Proteolytic mechanisms in amyloid-beta metabolism: therapeutic implications for Alzheimer's disease

Potential Roles of Hypoxia-Inducible Factor-1 in Alzheimer's Disease: Beneficial or Detrimental?

The major pathological characteristics of Alzheimer's disease (AD) include senile plaques and neurofibrillary tangles (NFTs), which are mainly composed of aggregated amyloid-beta (Aβ) peptide and hyperphosphorylated tau protein, respectively. The excessive production of reactive oxygen species (ROS) and neuroinflammation are crucial contributing factors to the pathological mechanisms of AD. Hypoxia-inducible factor-1 (HIF-1) is a transcription factor critical for tissue adaption to low-oxygen tension. Growing evidence has suggested HIF-1 as a potential therapeutic target for AD; conversely, other experimental findings indicate that HIF-1 induction contributes to AD pathogenesis. These previous findings thus point to the complex, even contradictory, roles of HIF-1 in AD. In this review, we first introduce the general pathogenic mechanisms of AD as well as the potential pathophysiological roles of HIF-1 in cancer, immunity, and oxidative stress. Based on current experimental evidence in the literature, we then discuss the possible beneficial as well as detrimental mechanisms of HIF-1 in AD; these sections also include the summaries of multiple chemical reagents and proteins that have been shown to exert beneficial effects in AD via either the induction or inhibition of HIF-1.

Therapeutic Efficacy of Extracellular Vesicles Derived from Stem Cell for Alzheimer's Disease: A Meta-Analysis Study

BACKGROUND

Alzheimer's disease (AD) poses a significant public health challenge, increasingly affecting patients' finances, mental health, and functional abilities as the global population ages. Stem cell-derived extracellular vesicles (SC-EVs) have emerged as a promising cell-free therapeutic approach for AD, although their precise mechanisms remain unclear. This meta-analysis aims to evaluate the effectiveness of SC-EVs in treating AD.

METHODS

We systematically searched PubMed, EMBASE, and Web of Science databases up to December 31, 2023, identifying studies investigating SC-EVs therapy in AD rodent models. Outcome measures included Morris water maze and Y maze tests, β-amyloid pathology, and inflammatory markers. Statistical analyses utilized Stata 15.1 and R software.

RESULTS

This meta-analysis of 16 studies (2017-2023, 314 animals) demonstrates significant efficacy of SC-EVs therapy in AD models. Pooled analyses demonstrated that SC-EVs therapy significantly increased the learning function as measured by Morris water maze tests (MWM) by -1.83 (95% CI = -2.51 to -1.15, p < 0.0001), Y maze test by 1.66 (95% CI = 1.03 to 2.28, p < 0.0001), decreased Aβ plaques in the hippocampal by -2.10 (95% CI = -2.96 to -1.23, p < 0.0001), and proinflammatory cytokines Tumor necrosis factor alpha (TNFα) by -2.61 (95% CI = -4.87 to -0.35, p < 0.05), Interleukin-1 beta (IL-1β) by -2.37 (95% CI = -3.68 to -1.05, p < 0.001).

CONCLUSIONS

SC-EVs therapy shows promise in enhancing cognitive function and mitigating AD progression in preclinical models. Future research should focus on standardizing methodologies and comparing SC-EVs isolation techniques and dosing strategies to facilitate clinical translation.

Enzyme-Responsive Supramolecular Self-Assembly in Small Amphiphiles

Enzyme-responsive molecular assemblies have recently made remarkable progress, owing to their widespread applications. As a class of catalysts with high specificity and efficiency, enzymes play a critical role in producing new molecules and maintaining metabolic stability in living organisms. Therefore, the study of enzyme-responsive assembly aids in understanding the origin of life and the physiological processes occurring within living bodies, contributing to further advancements across various disciplines. In this Review, we summarize three kinds of enzyme-responsive assembly systems in amphiphiles: enzyme-triggered assembly, disassembly, and structural transformation. Furthermore, motivated by the fact that biological macromolecules and complex structures all originated with small molecules, our focus lies on the small amphiphiles (e.g., peptides, surfactants, fluorescent molecules, and drug molecules). We also provide an outlook on the potential of enzyme-responsive assembly systems for biomimetic development and hope this Review will attract more attention to this emerging research branch at the intersection of assembly chemistry and biological science.

Polysaccharides from Basella alba Protect Post-Mitotic Neurons against Cell Cycle Re-Entry and Apoptosis Induced by the Amyloid-Beta Peptide by Blocking Sonic Hedgehog Expression

The amyloid-beta peptide (Aβ) is the neurotoxic component in senile plaques of Alzheimer's disease (AD) brains. Previously we have reported that Aβ toxicity is mediated by the induction of sonic hedgehog (SHH) to trigger cell cycle re-entry (CCR) and apoptosis in post-mitotic neurons. Basella alba is a vegetable whose polysaccharides carry immunomodulatory and anti-cancer actions, but their protective effects against neurodegeneration have never been reported. Herein, we tested whether polysaccharides derived from Basella alba (PPV-6) may inhibit Aβ toxicity and explored its underlying mechanisms. In differentiated rat cortical neurons, Aβ25-35 reduced cell viability, damaged neuronal structure, and compromised mitochondrial bioenergetic functions, all of which were recovered by PPV-6. Immunocytochemistry and western blotting revealed that Aβ25-35-mediated induction of cell cycle markers including cyclin D1, proliferating cell nuclear antigen (PCNA), and histone H3 phosphorylated at Ser-10 (p-Histone H3) in differentiated neurons was all suppressed by PPV-6, along with mitigation of caspase-3 cleavage. Further studies revealed that PPV-6 inhibited Aβ25-35 induction of SHH; indeed, PPV-6 was capable of suppressing neuronal CCR and apoptosis triggered by the exogenous N-terminal fragment of sonic hedgehog (SHH-N). Our findings demonstrated that, in the fully differentiated neurons, PPV-6 exerts protective actions against Aβ neurotoxicity via the downregulation of SHH to suppress neuronal CCR and apoptosis.

Evaluation and targeting of amyloid precursor protein (APP)/amyloid beta (Aβ) axis in amyloidogenic and non-amyloidogenic pathways: A time outside the tunnel

In Alzheimer disease (AD), amyloid precursor protein (APP) and production of amyloid beta (Aβ) which is generated by amyloidogenic pathway is implicated in neurotoxicity and neuronal cell deaths. However, physiological Aβ level is essential to improves neuronal survival, attenuates neuronal apoptosis and has neuroprotective effect. In addition, physiological APP level has neurotrophic effect on the central nervous system (CNS). APP has a critical role in the brain growth and development via activation of long-term potentiation (LTP) and acceleration of neurite outgrowth. Moreover, APP is cleaved by α secretase to form a neuroprotective soluble APP alpha (sAPPα) in non-amyloidogenic pathway. Consequently, this mini-review purposes to highlight the possible beneficial role of APP and Aβ. In addition, this mini-review discussed the modulation of APP processing and Aβ production.

Cell primitive-based biomimetic nanomaterials for Alzheimer's disease targeting and therapy

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, which is not just confined to the older population. Although developments have been made in AD treatment, various limitations remain to be addressed. These are partly contributed by biological hurdles, such as the blood-brain barrier and peripheral side effects, as well as by lack of carriers that can efficiently deliver the therapeutics to the brain while preserving their therapeutic efficacy. The increasing AD prevalence and the unavailability of effective treatments have encouraged researchers to develop improved, convenient, and affordable therapies. Functional materials based on primitive cells and nanotechnology are emerging as attractive therapeutics in AD treatment. Cell primitives possess distinct biological functions, including long-term circulation, lesion site targeting, and immune suppression. This review summarizes the challenges in the delivery of AD drugs and recent advances in cell primitive-based materials for AD treatment. Various cell primitives, such as cells, extracellular vesicles, and cell membranes, are presented together with their distinctive biological functions and construction strategies. Moreover, future research directions are discussed on the basis of foreseeable challenges and perspectives.

Intranasally Co-administered Berberine and Curcumin Loaded in Transfersomal Vesicles Improved Inhibition of Amyloid Formation and BACE-1

Selective permeability of the blood-brain barrier restricts the treatment efficacy of neurologic diseases. Berberine (BBR) and curcumin (CUR)-loaded transferosomes (TRANS) were prepared for the effective management of Alzheimer's disease (AD). The study involved the syntheses of BBR-TRANS, CUR-TRANS, and BBR-CUR-TRANS by the film hydration method. Vesicles were characterized to ensure the formation of drug-loaded vesicles and their in vivo performance. The particle sizes of BBR-TRANS, CUR-TRANS, and BBR-CUR-TRANS were 139.2 ± 7, 143.4 ± 8, and 165.3 ± 6.5 nm, respectively. The presence of diffused rings in the SED image indicates the crystalline nature of the payload. Low surface roughness in an AFM image could be associated with the presence of a surface lipid. BBR-CUR-TRANS showed 41.03 ± 1.22 and 47.79 ± 3.67% release of BBR and 19.22 ± 1.47 and 24.67 ± 1.94% release of CUR, respectively, in phosphate buffer saline (pH 7.4) and acetate buffer (pH 4.0). Formulations showed sustained release of both loaded drugs. BBR-TRANS, CUR-TRANS, and BBR-CUR-TRANS exhibited a lower percentage of hemolysis than pure BBR and CUR, indicating the safety of the payload from delivery vesicles. Lower percentages of binding were recorded from BBR-CUR-TRANS than BBR-TRANS and CUR-TRANS. Acetylcholinesterase inhibition activity of the prepared transferosomes was greater than that of pure drugs, which are thought to have good cellular penetration. The spatial memory was improved in treated mice models. The level of malondialdehyde decreased in AD animals treated with BBR-TRANS, CUR-TRANS, and BBR-CUR-TRANS, respectively, as compared to the scopolamine-induced AD animals. BBR-CUR-TRANS-treated animals showed the highest decrease in the NO level. The catalase level was significantly restored in scopolamine-intoxicated animals treated with BBR-TRANS, CUR-TRANS, and BBR-CUR-TRANS. The immunohistochemistry result suggested that the BBR-TRANS, CUR-TRANS, and BBR-CUR-TRANS have significantly decreased the regulation of expression of BACE-1 through antioxidant activity. In conclusion, the study highlights the utility of formulated transferosomes as promising carriers for the co-delivery of drugs to the brain.

Amyloid Beta in Aging and Alzheimer's Disease

Alzheimer's disease (AD), is a progressive neurodegenerative disease that affects behavior, thinking, learning, and memory in elderly individuals. AD occurs in two forms, early onset familial and late-onset sporadic; genetic mutations in PS1, PS2, and APP genes cause early onset familial AD, and a combination of lifestyle, environment and genetic factors causes the late-onset sporadic form of the disease. However, accelerated disease progression is noticed in patients with familial AD. Disease-causing pathological changes are synaptic damage, and mitochondrial structural and functional changes, in addition to increased production and accumulation of phosphorylated tau (p-tau), and amyloid beta (Aβ) in the affected brain regions in AD patients. Aβ is a peptide derived from amyloid precursor protein (APP) by proteolytic cleavage of beta and gamma secretases. APP is a glycoprotein that plays a significant role in maintaining neuronal homeostasis like signaling, neuronal development, and intracellular transport. Aβ is reported to have both protective and toxic effects in neurons. The purpose of our article is to summarize recent developments of Aβ and its association with synapses, mitochondria, microglia, astrocytes, and its interaction with p-tau. Our article also covers the therapeutic strategies that reduce Aβ toxicities in disease progression and discusses the reasons for the failures of Aβ therapeutics.

Serum metabolites associated with brain amyloid beta deposition, cognition and dementia progression

Metabolomics in the Alzheimer's Disease Neuroimaging Initiative cohort provides a powerful tool for mapping biochemical changes in Alzheimer's disease, and a unique opportunity to learn about the association between circulating blood metabolites and brain amyloid-β deposition in Alzheimer's disease. We examined 140 serum metabolites and their associations with brain amyloid-β deposition, cognition and conversion from mild cognitive impairment to Alzheimer's disease in the Alzheimer's Disease Neuroimaging Initiative. Processed [18F] Florbetapir PET images were used to perform a voxel-wise statistical analysis of the effect of metabolite levels on amyloid-β accumulation across the whole brain. We performed a multivariable regression analysis using age, sex, body mass index, apolipoprotein E ε4 status and study phase as covariates. We identified nine metabolites as significantly associated with amyloid-β deposition after multiple comparison correction. Higher levels of one acylcarnitine (C3; propionylcarnitine) and one biogenic amine (kynurenine) were associated with decreased amyloid-β accumulation and higher memory scores. However, higher levels of seven phosphatidylcholines (lysoPC a C18:2, PC aa C42:0, PC ae C42:3, PC ae C44:3, PC ae C44:4, PC ae C44:5 and PC ae C44:6) were associated with increased brain amyloid-β deposition. In addition, higher levels of PC ae C44:4 were significantly associated with lower memory and executive function scores and conversion from mild cognitive impairment to Alzheimer's disease dementia. Our findings suggest that dysregulation of peripheral phosphatidylcholine metabolism is associated with earlier pathological changes noted in Alzheimer's disease as measured by brain amyloid-β deposition as well as later clinical features including changes in memory and executive functioning. Perturbations in phosphatidylcholine metabolism may point to issues with membrane restructuring leading to the accumulation of amyloid-β in the brain. Additional studies are needed to explore whether these metabolites play a causal role in the pathogenesis of Alzheimer's disease or if they are biomarkers for systemic changes during preclinical phases of the disease.

[Donepezil Reduces Amyloid Precursor Protein Endocytosis by Resulting from Increase in the Expression of Sorting Nexin Protein 33]

Donepezil, the most widely used drug for the treatment of Alzheimer's disease (AD), is an acetylcholinesterase (AChE) inhibitor and is thought to improve cognition by stimulating cholinergic neurotransmission. However, no correlation has yet been established between the inhibitory role of AChE inhibitors and their therapeutic effects when used in AD patients. The cleavage pathway of amyloid precursor protein (APP) includes amyloidgenic (β, γ-cleavage) and non-amyloidgenic (α-cleavage) pathways. The intracellular transportation of APP is important in determining these cleavage pathways. It has been suggested that sorting nexin (SNX) family proteins regulates the intracellular transport of APP, thereby enhancing α-cleavage. In this study, we examined the effects of donepezil on SNX33 expression changes and APP processing in primary cultures of fetal rat cortical neurons. While donepezil treatment increased the levels of SNX33 expression and soluble APPα (sAPPα) in culture media, no changes were observed regarding full-length APP expression in the cell lysate. Donepezil also reduced the release of amyloid β (Aβ) into culture media in a concentration- and time-dependent manner. This reduction was not affected by acetylcholine receptor antagonists. The membrane surface expression of APP was elevated by donepezil. Furthermore, SNX knockdown by antisense morpholino oligos prevented the effects of donepezil. These results indicated that donepezil increased APP expression at the surface of the plasma membrane by decreasing APP endocytosis through upregulation of SNX33, suggesting donepezil might stimulate the non-amyloidogenic pathway. This new mechanism of action for the currently used anti-AD drug may provide a valuable basis for future drug discovery.

The p53/NF-kappaB-dependent induction of sestrin2 by amyloid-beta peptides exerts antioxidative actions in neurons

Accumulation of senile plaques mainly composed of neurotoxic amyloid-beta peptide (Aβ) is a pathological hallmark of Alzheimer's disease (AD). Sestrin2 inducible by various types of stressors is known to promote autophagy and exert antioxidative effects. In this work, we revealed the molecular mechanisms underlying Aβ induction of sestrin2 and tested whether antioxidation, in addition to autophagy regulation, also contributes to its neuroprotective effects in primary rat cortical neurons. We found that Aβ25-35 triggered nuclear translocation of p65 and p50, two subunits of nuclear factor-kappaB (NF-κB), and p53. Aβ25-35-induced sestrin2 expression was abolished by the p65 siRNA, the NF-κB inhibitor SN50, and the p53 inhibitor pifithrin-alpha (PFT-α). Further, Aβ25-35 enhanced binding of p50 and p53 to sestrin2 gene promoter that was abolished respectively by the p50 shRNA and PFT-α. Both p50 shRNA and PFT-α attenuated Aβ25-35-induced expression as well as nuclear translocation of all three transcription factors, namely p65, p50, and p53. Interestingly, p50 binding to the promoters of its target genes required p53 activity, whereas p50 also negatively regulated p53 binding to its target sequences. Suppression of sestrin2 expression by siRNA enhanced Aβ25-35- and Aβ1-42-induced production of reactive oxygen species (ROS), lipid peroxidation, and formation of 8-hydroxy-2-deoxyguanosine (8-OH-dG). In contrast, overexpression of the sestrin2 N-terminal or C-terminal fragments neutralized Aβ25-35-induced ROS production. We concluded that Aβ-induced sestrin2 contributing to antioxidant effects in neurons is in part mediated by p53 and NF-κB, which also mutually affect the expression of each other.

Examining the effects of ovarian hormone loss and diet-induced obesity on Alzheimer's disease markers of amyloid-β production and degradation

After menopause, women experience declines in ovarian sex hormones, an event that has recently been associated with increased amyloid-β peptides, a main feature of Alzheimer's disease. Diet-induced insulin resistance also increases amyloid-β peptides; however, whether this process is exacerbated with ovarian sex hormone loss remains unknown. Female C57BL6/J mice received either bilateral ovariectomy (OVX; n = 20) or remained intact (n = 20) at 24 wk of age and were placed on either a low- or high-fat diet (LFD, n = 10 for OVX and intact; HFD, n = 10 for OVX and intact) for 10 wk. Independently, OVX led to increases in the amyloidogenic marker, soluble amyloid precursor protein β (sAPPβ). The HFD in combination with OVX led to lower insulin degrading enzyme (IDE) protein content and activity in the prefrontal cortex, indicative of decreased amyloid-β degradation; however, no differences in amyloid-β content were observed. Data from this study provide novel evidence of independent effects of peripheral insulin resistance and ovarian sex hormone loss in decreasing brain markers of amyloid-β degradation. Furthermore, findings indicate how the loss of ovarian sex hormones can promote the formation of amyloidogenic APP cleavage products, independent of diet-induced insulin resistance.NEW & NOTEWORTHY This study provides novel insight into the effect of peripheral insulin resistance and ovarian hormone loss in decreasing brain markers of amyloid-β degradation. Results demonstrate that ovarian hormone loss through ovariectomy increased the amyloidogenic marker, sAPPβ, while the high-fat diet in combination with ovariectomy led to lower IDE protein content and activity in the prefrontal cortex, indicative of decreased amyloid-β degradation. These original results provide important information for future targets in early AD pathogenesis.

Is It the Twilight of BACE1 Inhibitors?

β-secretase (BACE1) has been regarded as a prime target for the development of amyloid beta (Aβ) lowering drugs in the therapy of Alzheimer´s disease (AD). Although the enzyme was discovered in 1991 and helped to formulate the Aβ hypothesis as one of the very important features of AD etiopathogenesis, progress in AD treatment utilizing BACE1 inhibitors has remained limited. Moreover, in the last years, major pharmaceutical companies have discontinued clinical trials of five BACE1 inhibitors that had been strongly perceived as prospective. In our review, the Aβ hypothesis, the enzyme, its functions, and selected substrates are described. BACE1 inhibitors are classified into four generations. Those that underwent clinical trials displayed adverse effects, including weight loss, skin rashes, worsening of neuropsychiatric symptoms, etc. Some inhibitors could not establish a statistically significant risk-benefit ratio, or even scored worse than placebo. We still believe that drugs targeting BACE1 may still hide some potential, but a different approach to BACE1 inhibition or a shift of focus to modulation of its trafficking and/or post-translational modification should now be followed.

Molecular Dynamics Simulations of Acetylcholinesterase – Beta-Amyloid Peptide Complex

Alzheimer’s Disease (AD) is a neurodegenerative disorder with severe consequences and lethal outcome. One of the pathological hallmarks of the disease is the formation of insoluble intercellular beta-Amyloid (Aβ) plaques. The enzyme ACetylcholinEsterase (AChE) promotes and accelerates the aggregation of toxic Aβ protofibrils progressively converted into plaques. The Peripheral Anionic Site (PAS), part of the binding gorge of AChE, is one of the nucleation centers implicated in the Aβ aggregation. In this study, the Aβ peptide was docked into the PAS and the stability of the formed complex was investigated by molecular dynamics simulation for 1 μs (1000 ns). The complex was stable during the simulation. Apart from PAS, the Aβ peptide makes several additional contacts with AChE. The main residence area of Aβ on the surface of AChE is the region 344-361. This region is next to PAS but far enough to be sterically hindered by dual-site binding AChE inhibitors.

Biosensing: The best alternative for conventional methods in detection of Alzheimer's disease biomarkers

Conceivably the imperative reason for the absence of appropriate treatment for Alzheimer's disease (AD) is the late onset of clinical symptoms followed by late treatment. Specific biomarkers play a vital role in this area. The amyloid-beta peptide, tau protein and micRNA, are the most important biomarker associated in AD. There are many routine methods for identifying these biomarkers which molecular based methods with a high accuracy and sensitivity have been considered. These methods have some limitations such as; false positive and negative results, problem on the interpretation, complexity and time-consuming, high cost instruments and etc. To overcome these limitations, bioassays were developed extensively. There exist a multitude of possible applications for Alzheimer's disease biomarkers by using biosensors. This review mainly focuses on major biomarkers in Alzheimer's disease, routine and old methods in identifying biomarkers of AD and their advantages and limitation, and biosensors to the identification of amyloid beta, tau protein and micRNAs biomarkers. Furthermore, evaluation the strengths and weaknesses of the developed bioassays and introduce leading challenges are considered in this review.

Enzymatic Noncovalent Synthesis

Enzymatic reactions and noncovalent (i.e., supramolecular) interactions are two fundamental nongenetic attributes of life. Enzymatic noncovalent synthesis (ENS) refers to a process where enzymatic reactions control intermolecular noncovalent interactions for spatial organization of higher-order molecular assemblies that exhibit emergent properties and functions. Like enzymatic covalent synthesis (ECS), in which an enzyme catalyzes the formation of covalent bonds to generate individual molecules, ENS is a unifying theme for understanding the functions, morphologies, and locations of molecular ensembles in cellular environments. This review intends to provide a summary of the works of ENS within the past decade and emphasize ENS for functions. After comparing ECS and ENS, we describe a few representative examples where nature uses ENS, as a rule of life, to create the ensembles of biomacromolecules for emergent properties/functions in a myriad of cellular processes. Then, we focus on ENS of man-made (synthetic) molecules in cell-free conditions, classified by the types of enzymes. After that, we introduce the exploration of ENS of man-made molecules in the context of cells by discussing intercellular, peri/intracellular, and subcellular ENS for cell morphogenesis, molecular imaging, cancer therapy, and other applications. Finally, we provide a perspective on the promises of ENS for developing molecular assemblies/processes for functions. This review aims to be an updated introduction for researchers who are interested in exploring noncovalent synthesis for developing molecular science and technologies to address societal needs.

Mesenchymal stem cell-derived exosome: a promising alternative in the therapy of Alzheimer's disease

Alzheimer's disease (AD) has been a devastating public health with the development of global aging. Approaches for reducing the current AD epidemic are becoming a primary focus of human healthcare due to the lack of achieved lasting and complete remission strategies to treat AD with the characteristics of heterogeneity and complexity. Exosomes, which is the new emerging approach to intercellular communication, provide novel perspective on identified therapeutic strategies of AD. Mesenchymal stem cell-derived exosomes (MSC-exos) are emerging to be an appealing therapeutic tool for AD, with the donor-derived properties and the characteristics of minimal immunogenicity, effortless storage, nature delivery vehicles, and low risks of tumor formation based on the previous researches. In this review, we elaborate the mechanism of MSC-exos in the treatment of AD and discuss limitations in the clinical application.

Aβ40 Oligomers Promote Survival and Early Neuronal Differentiation of Dentate Gyrus-Isolated Precursor Cells Through Activation of the Akt Signaling Pathway

The amyloid beta-peptide (Aβ) is the low-abundance product of amyloid precursor protein (APP), which is produced lifelong in the healthy brain. The functional properties of Aβ40 and Aβ42 peptides have not been completely elucidated to date. Although, several studies suggest that these peptides have a number of neurotrophic and neurotoxic properties, respectively. Interestingly, low concentrations of Aβ40 and Aβ42 regulate synaptic plasticity and improve cognitive functions, whereas the accumulation of Aβ42, coupled with the effects of age, can cause dysregulation of synaptic function, as is shown in Alzheimer's disease. Additionally, several studies suggest that both peptides, Aβ40 and Aβ42, are involved in neurogenic processes; however, these results are still controversial. Moreover, existing data indicate a direct relationship between the physicochemical characteristics of the peptides and their effects. Herein, we evaluated the effect of Aβ40 oligomers on hippocampal precursor cells isolated from the dentate gyrus of adult female C57Bl6 mice (mADGPCs). To this end, mADGPCs were treated with nanomolar and micromolar range concentrations of oligomeric forms of Aβ40 for 24, 48, and 72 h to evaluate their effects on several events in the neurogenic process in vitro, including viability, proliferation, and early differentiation. The results indicate that Aβ40 favors mADGPC proliferation, survival, and neuronal differentiation following a mechanism that involves activation of the Akt signaling pathway. Thus, this study provides evidence about the positive effects of Aβ40 oligomers on the neurogenic process in adult mouse hippocampal precursor cells in vitro.

Neuro biomarker levels measured with high-sensitivity digital ELISA differ between serum and plasma

Aim: Digital ELISA-based assays for blood biomarkers of neurological disease are on the verge of clinical use. Here, we aimed to determine whether different preanalytical blood processing techniques influence results. Materials & methods: Concentrations of neurofilament light chain (NfL), Tau and amyloid beta (Aβ) were measured in human plasma and serum specimens using digital ELISA and compared between blood products. Measured levels of NfL were highly equivalant between serum and plasma in all analyses, however, measured levels of Tau and Aβ were consistently lower in serum relative to plasma. Conclusion: Tau and Aβ are likely lost during clotting in serum preparations, and should be assayed in plasma to get an accurate measure of circulating levels.

Donepezil modulates amyloid precursor protein endocytosis and reduction by up-regulation of SNX33 expression in primary cortical neurons

Donepezil, a therapeutic drug for Alzheimer’s disease, ameliorates cognitive dysfunction through selective inhibition of acetylcholinesterase. However, recent studies have also reported off-target effects of donepezil that likely contribute to its therapeutic effects. In this study, we investigated the (i) role of donepezil in amyloid precursor protein (APP) processing and (ii) involvement of sorting nexin protein 33 (SNX33), a member of the sorting nexin protein family, in this processing. Results showed that donepezil induces an increase in SNX33 expression in primary cortical neurons. The secretion of sAPPα in culture media increased, whereas the expression of full-length APP in the cell lysate remained unchanged. Exposure of cortical cultures to donepezil led to a decrease in amyloid β (Aβ) protein levels in a concentration- and time-dependent manner. This decrease was not affected by concomitant treatment with acetylcholine receptor antagonists. SNX33 knockdown by target-specific morpholino oligos inhibited the effects of donepezil. Donepezil treatment increased cell membrane surface expression of APP in SNX33 expression-dependent manner. These results suggested that donepezil decreases the level of Aβ by increasing SNX33 expression and APP cleavage by α-secretase in cortical neurons.

Natural silibinin modulates amyloid precursor protein processing and amyloid-β protein clearance in APP/PS1 mice

Silibinin has been shown to attenuate cognitive dysfunction and inhibit amyloid-beta (Aβ) aggregation in Alzheimer's disease (AD) models. However, the underlying mechanism by which silibinin improves cognition remains poorly understood. In this study, we investigated the effect of silibinin on β-secretase levels, Aβ enzymatic degradation, and oxidative stress in the brains of APP/PS1 mice with cognitive impairments. Oral administration of silibinin for 2 months significantly attenuated the cognitive deficits of APP/PS1 mice in the Y-maze test, novel object recognition test, and Morris water maze test. Biochemical analyses revealed that silibinin decreased Aβ deposition and the levels of soluble Aβ1-40/1-42 in the hippocampus by downregulating APP and BACE1 and upregulating NEP in APP/PS1 mice. In addition, silibinin decreased the MDA content and increased the activities of the antioxidant enzymes CAT, SOD, and NO. Based on our findings, silibinin is a potentially promising agent for preventing AD-associated Aβ pathology.

Potential of South African medicinal plants targeting the reduction of Aβ42 protein as a treatment of Alzheimer's disease

ETHNOPHARMACOLOGICAL RELEVANCE

Twenty South African medicinal plant species were selected by conducting a literature review based on the relevant information of their reported traditional medicinal uses and scientific reports against Alzheimer's disease, dementia, anxiety, mental illness, depression, acetylcholinesterase inhibition, headache, epilepsy, convulsion, hysteria, and sedative effects.

AIM OF STUDY

The goal of this study was to investigate the biological activity of the traditionally used medicinal plant extracts against Alzheimer's disease by in vitro screening of the extracts to determine their potential to decrease levels of Aβ42 protein.

MATERIAL AND METHODS

Different plant parts (leaves, stem, bark, and stalks) of twenty selected plants were collected from the Manie van der Schijff Botanical Garden, University of Pretoria. Plant parts were dried, ground and then extracted using DCM:MeOH (1:1). We measured the levels of β-amyloid precursor protein proteolytic products in HeLa cells stably transfected with APP carrying the Swedish mutation using ELISA.

RESULTS

Of 33 plant extract 10 (30.3%) were found active based on the potential to significantly reduce the production of Aβ42. Amongst them extracts of leaves of Xysmalobium undulatum (Apocynaceae), leaves of Cussonia paniculata (Araliaceae) and leaves of Schotia brachypetala (Fabaceae) potently decreased the production of Aβ42 by 77.3 ± 0.5%, 57.5 ± 1.3%, and 44.8 ± 0.1%, respectively. X. undulatum and S. brachypetala enhanced non-amyloidogenic processing of β-amyloid precursor protein, thereby decreasing Aβ42 level. We also showed that C. paniculata induced the decrease of Aβ42 level through inhibiting APP processing. In addition, we isolated two cardenolides, compound [A] and [B], from X. undulatum and found that they potently decreased the Aβ42 production.

CONCLUSION

These data suggest that the extract of X. undulatum, C. paniculata, and S. brachypetala have potential to be developed for Alzheimer's disease treatment. These active extracts and compounds are considered for further studies which examine their efficacy towards the reduction of Aβ42 through inhibiting APP process.

Korean Thistle (Cirsium japonicum var. maackii (Maxim.) Matsum.): A Potential Dietary Supplement against Diabetes and Alzheimer's Disease

In the search for natural products having a dual inhibitory action on diabetes and Alzheimer’s disease, this study investigated the activity of different parts of Korean thistle (Cirsium japonicum var. maackii (Maxim.) Matsum), and its fractional constituents by in vitro enzymatic and in silico molecular docking studies. Cirsium maackii has been used as a traditional medicine for the treatment of several diseases. The ethyl acetate and dichloromethane fractions of a leaf extract showed α-glucosidase and BACE1 inhibitory activity, respectively. Furthermore, the isolated compound, luteolin, exhibited concentration-dependent non-competitive inhibition against both α-glucosidase and BACE1 (IC₅₀ = 51.27 ± 1.23 and 13.75 ± 0.26 μM; K_i value = 52.04 and 14.76 μM, respectively). Moreover, docking studies showed that luteolin formed a strong hydrogen bond with the peripheral binding amino acid residues, and hydrophobic interactions with the α-glucosidase and BACE1 enzymes. Therefore, Korean thistle may act as an important dietary supplement against diabetes and Alzheimer’s disease, especially the leaves, because of the preponderance of the active component, luteolin, making Korean thistle a promising candidate for more detailed in vitro and in vivo studies.

MicroRNA-128 knockout inhibits the development of Alzheimer's disease by targeting PPARγ in mouse models

Alzheimer's disease (AD) is a great threat for the health and life of elderly people. MicroRNA-128 (miR-128) has been reported to be abnormally expressed in the brain of AD patients and associated with the pathogenesis of AD. Our study aimed to have a deep insight into the roles and molecular basis of miR-128 in the development and progression of AD. The cognitive ability and exploratory behaviors were assessed by morris water maze and open-field tests, respectively. The concentrations of amyloid-β (Aβ) 40, Aβ 42, tumor necrosis factor (TNF)-α, interleukin (IL)-1β and IL-10 and activity of β-secretase and α-secretase were determined by corresponding ELISA commercial kits. RT-qPCR assay was performed to detect miR-128 level and the mRNA expression of peroxisome proliferator-activated receptor gamma (PPARγ), ionized calcium-binding adaptor molecule 1 (Iba1) and glial fibrillary acidic protein (GFAP). Western blot assay was conducted to determine protein expression of PPARγ, amyloid precursor protein (APP), β-APP cleaving enzyme (BACE1), sAPPα and sAPPβ. The effect of miR-128 and PPARγ on amyloid plaque formation was assessed by immunohistochemistry assay. PPARγ mean optical density was determined by immunofluorescence assay. The interaction between miR-128 and PPARγ were validated by bioinformatics analysis and luciferase reporter assay. We found AD mice showed AD-like performance and an increased cerebral cortex Aβ production. MiR-128 expression was upregulated and PPARγ expression was downregulated in cerebral cortex of AD mice. Moreover, PPARγ was a target of miR-128. Additionally, miR-128 knockout or PPARγ upregulation inhibited AD-like performances, amyloid plaque formation, Aβ generation, APP amyloidogenic processing and inflammatory responses in AD mice, while these effects of miR-128 knockout were abrogated by PPARγ inhibitor. The results indicated MiR-128 knockout weakened AD-like performances, and reduced Aβ production and inflammatory responses by targeting PPARγ in AD mice.

O-GlcNAcylation and neuronal energy status: Implications for Alzheimer's disease

Since the first clinical case reported more than 100 years ago, it has been a long and winding road to demystify the initial pathological events underling the onset of Alzheimer's disease (AD). Fortunately, advanced imaging techniques extended the knowledge regarding AD origin, being well accepted that a decline in brain glucose metabolism occurs during the prodromal phases of AD and is aggravated with the progression of the disease. In this sense, in the last decades, the post-translational modification O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) has emerged as a potential causative link between hampered brain glucose metabolism and AD pathology. This is not surprising taking into account that this dynamic post-translational modification acts as a metabolic sensor that links glucose metabolism to normal neuronal functioning. Within this scenario, the present review aims to summarize the current understanding on the role of O-GlcNAcylation in neuronal physiology and AD pathology, emphasizing the close association of this post-translational modification with the emergence of AD-related hallmarks and its potential as a therapeutic target.

The neuroprotective role of melatonin in neurological disorders

Melatonin is a neurohormone secreted from the pineal gland and has a wide-ranging regulatory and neuroprotective role. It has been reported that melatonin level is disturbed in some neurological conditions such as stroke, Alzheimer's disease, and Parkinson's disease, which indicates its involvement in the pathophysiology of these diseases. Its properties qualify it to be a promising potential therapeutic neuroprotective agent, with no side effects, for some neurological disorders. This review discusses and localizes the effect of melatonin in the pathophysiology of some diseases.

Unveiling Brain Aβ Heterogeneity Through Targeted Proteomic Analysis

Amyloid β (Aβ) is the major constituent of the brain deposits found in parenchymal plaques and cerebral blood vessels of patients with Alzheimer's disease (AD). Besides classic full-length peptides, biochemical analyses of brain deposits have revealed high degree of Aβ heterogeneity likely resulting from the action of multiple proteolytic enzymes. This chapter describes a sequential extraction protocol allowing the differential fractionation of soluble and deposited Aβ species taking advantage of their differential solubility properties. Soluble Aβ is extracted by water-based buffers like phosphate-buffered saline-PBS-whereas pre-fibrillar and fibrillar deposits, usually poorly soluble in PBS, are extractable in detergent containing solutions or more stringent conditions as formic acid. The extraction procedure is followed by the biochemical identification of the extracted Aβ species using Western blot and a targeted proteomic analysis which combines immunoprecipitation with MALDI-ToF mass spectrometry. This approach revealed the presence of numerous C- and N-terminal truncated Aβ species in addition to Aβ1-40/42. Notably, the more soluble C-terminal cleaved fragments constitute a main part of PBS homogenates. On the contrary, N-terminal truncated species typically require more stringent conditions for the extraction in agreement with their lower solubility and enhanced aggregability. Detailed assessment of the molecular diversity of Aβ species composing interstitial fluid and amyloid deposits at different disease stages, as well as the evaluation of the truncation profile during various pharmacologic approaches will provide a comprehensive understanding of the still undefined contribution of Aβ truncations to AD pathogenesis and their potential as novel therapeutic targets.

Intranasal Lactoferrin Enhances α-Secretase-Dependent Amyloid Precursor Protein Processing via the ERK1/2-CREB and HIF-1α Pathways in an Alzheimer's Disease Mouse Model

Growing evidence suggests that lactoferrin (Lf), an iron-binding glycoprotein, is a pleiotropic functional nutrient. In addition, Lf was recently implicated as a neuroprotective agent. These properties make Lf a valuable therapeutic candidate for the treatment of Alzheimer's disease (AD). However, the mechanisms regulating the physiological roles of Lf in the pathologic condition of AD remain unknown. In the present study, an APPswe/PS1DE9 transgenic mouse model of AD was used. We explored whether intranasal human Lf (hLf) administration could reduce β-amyloid (Aβ) deposition and ameliorate cognitive decline in this AD model. We found that hLf promoted the non-amyloidogenic metabolism of amyloid precursor protein (APP) processing through activation of α-secretase a-disintegrin and metalloprotease10 (ADAM10), resulting in enhanced cleavage of the α-COOH-terminal fragment of APP and the corresponding elevation of the NH2-terminal APP product, soluble APP-α (sAPPα), which consequently reduced Aβ generation and improved spatial cognitive learning ability in AD mice. To gain insight into the molecular mechanism by which Lf modulates APP processing, we evaluated the involvement of the critical molecules for APP cleavage and the signaling pathways in N2a cells stably transfected with Swedish mutant human APP (APPsw N2a cells). The results show that the ERK1/2-CREB and HIF-1α signaling pathways were activated by hLf treatment, which is responsible for the expression of induced ADAM10. Additional tests were performed before suggesting the potential use of hLf as an antioxidant and anti-inflammatory. These findings provide new insights into the sources and mechanisms by which hLf inhibits the cognitive decline that occurs in AD via activation of ADAM10 expression in an ERK1/2-CREB and HIF-1α-dependent manner.

Distinct amyloid precursor protein processing machineries of the olfactory system

Processing of amyloid precursor protein (APP) occurs through sequential cleavages first by β-secretase and then by the γ-secretase complex. However, abnormal processing of APP leads to excessive production of β-amyloid (Aβ) in the central nervous system (CNS), an event which is regarded as a primary cause of Alzheimer's disease (AD). In particular, gene mutations of the γ-secretase complex-which contains presenilin 1 or 2 as the catalytic core-could trigger marked Aβ accumulation. Olfactory dysfunction usually occurs before the onset of typical AD-related symptoms (eg, memory loss or muscle retardation), suggesting that the olfactory system may be one of the most vulnerable regions to AD. To date however, little is known about why the olfactory system is affected so early by AD prior to other regions. Thus, we examined the distribution of secretases and levels of APP processing in the olfactory system under either healthy or pathological conditions. Here, we show that the olfactory system has distinct APP processing machineries. In particular, we identified higher expressions levels and activity of γ-secretase in the olfactory epithelium (OE) than other regions of the brain. Moreover, APP c-terminal fragments (CTF) are markedly detected. During AD progression, we note increased expression of presenilin2 of γ-secretases in the OE, not in the OB, and show that neurotoxic Aβ*56 accumulates more quickly in the OE. Taken together, these results suggest that the olfactory system has distinct APP processing machineries under healthy and pathological conditions. This finding may provide a crucial understanding of the unique APP-processing mechanisms in the olfactory system, and further highlights the correlation between olfactory deficits and AD symptoms.

Meta-analysis of expression and function of neprilysin in Alzheimer's disease

Neprilysin (NEP) is one of the most important Aβ-degrading enzymes, and its expression and activity in Alzheimer's brain have been widely reported, but the results remain debatable. Thus, the meta-analysis was performed to elucidate the role of NEP in Alzheimer's disease (AD). The relevant case-control or cohort studies were retrieved according to our inclusion/exclusion criteria. Six studies with 123 controls and 141 AD cases, seven studies with 102 controls and 90 AD cases, and four studies with 93 controls and 132 AD cases were included in meta-analysis of NEP's protein, mRNA, and enzyme activity respectively. We conducted Meta regression to detect the sources of heterogeneity and further performed cumulative meta-analysis or subgroup analysis. Our meta-analysis revealed a significantly lower level of NEP mRNA (SMD=-0.44, 95%CI: -0.87, -0.00, p=0.049) in AD cases than in non-AD cases, and such pattern was not altered over time in the cumulative meta-analysis. However, the decrease of NEP protein (SMD=-0.18, 95%CI: -0.62, 0.25) and enzyme activity (SMD=-0.35, 95%CI: -1.03, 0.32) in AD cases did not pass the significance check, while the cumulative meta-analysis by average age showed the pooled effect became insignificant as adding the studies with younger subjects, which indicates that the protein expression and enzyme activity of NEP in the cortex are affected by age. Therefore, the present meta-analysis suggests the need of further investigation of roles of NEP in AD pathogenesis and treatment.

Aβ truncated species: Implications for brain clearance mechanisms and amyloid plaque deposition

Extensive parenchymal and vascular Aβ deposits are pathological hallmarks of Alzheimer's disease (AD). Besides classic full-length peptides, biochemical analyses of brain deposits have revealed high degree of Aβ heterogeneity likely resulting from the action of multiple proteolytic enzymes. In spite of the numerous studies focusing in Aβ, the relevance of N- and C-terminal truncated species for AD pathogenesis remains largely understudied. In the present work, using novel antibodies specifically recognizing Aβ species N-terminally truncated at position 4 or C-terminally truncated at position 34, we provide a clear assessment of the differential topographic localization of these species in AD brains and transgenic models. Based on their distinct solubility, brain N- and C-terminal truncated species were extracted by differential fractionation and identified via immunoprecipitation coupled to mass spectrometry analysis. Biochemical/biophysical studies with synthetic homologues further confirmed the different solubility properties and contrasting fibrillogenic characteristics of the truncated species composing the brain Aβ peptidome. Aβ C-terminal degradation leads to the production of more soluble fragments likely to be more easily eliminated from the brain. On the contrary, N-terminal truncation at position 4 favors the formation of poorly soluble, aggregation prone peptides with high amyloidogenic propensity and the potential to exacerbate the fibrillar deposits, self-perpetuating the amyloidogenic loop. Detailed assessment of the molecular diversity of Aβ species composing interstitial fluid and amyloid deposits at different disease stages, as well as the evaluation of the truncation profile during various pharmacologic approaches will provide a comprehensive understanding of the still undefined contribution of Aβ truncations to the disease pathogenesis and their potential as novel therapeutic targets.

Molecular Dynamics Simulations of Amyloid β-Peptide (1-42): Tetramer Formation and Membrane Interactions

The aggregation cascade and peptide-membrane interactions of the amyloid β-peptide (Aβ) have been implicated as toxic events in the development and progression of Alzheimer's disease. Aβ42 forms oligomers and ultimately plaques, and it has been hypothesized that these oligomeric species are the main toxic species contributing to neuronal cell death. To better understand oligomerization events and subsequent oligomer-membrane interactions of Aβ42, we performed atomistic molecular-dynamics (MD) simulations to characterize both interpeptide interactions and perturbation of model membranes by the peptides. MD simulations were utilized to first show the formation of a tetramer unit by four separate Aβ42 peptides. Aβ42 tetramers adopted an oblate ellipsoid shape and showed a significant increase in β-strand formation in the final tetramer unit relative to the monomers, indicative of on-pathway events for fibril formation. The Aβ42 tetramer unit that formed in the initial simulations was used in subsequent MD simulations in the presence of a pure POPC or cholesterol-rich raft model membrane. Tetramer-membrane simulations resulted in elongation of the tetramer in the presence of both model membranes, with tetramer-raft interactions giving rise to the rearrangement of key hydrophobic regions in the tetramer and the formation of a more rod-like structure indicative of a fibril-seeding aggregate. Membrane perturbation by the tetramer was manifested in the form of more ordered, rigid membranes, with the pure POPC being affected to a greater extent than the raft membrane. These results provide critical atomistic insight into the aggregation pathway of Aβ42 and a putative toxic mechanism in the pathogenesis of Alzheimer's disease.

Neuroprotective Effect of Nanodiamond in Alzheimer's Disease Rat Model: a Pivotal Role for Modulating NF-κB and STAT3 Signaling

Current therapeutic approaches of Alzheimer's disease (AD) are symptomatic and of modest efficacy, and there is no available effective cure or prevention of AD; hence, the need arise to search for neuroprotective agents to combat AD. The current study aimed at investigating the neuroprotective effect of nanodiamond (ND), adamantine-based nanoparticles, in aluminum-induced cognitive impairment in rats, an experimental model of AD. AD was induced by aluminum chloride (17 mg/kg, p.o. for 6 weeks) and confirmed by Morris water maze and Y-maze behavioral tests. Biochemical and histological analyses of the hippocampus were also performed. Aluminum-treated rats showed behavioral, biochemical, and histological changes similar to those associated with AD. ND improved learning and memory and reversed histological alterations. At the molecular levels, ND mitigated the increase of hippocampal beta-amyloid (Aβ42) and beta-site amyloid precursor protein cleaving enzyme-1 (BACE1) together with down-regulation of phosphorylated tau protein. It also modulated the excitatory glutamate neurotransmitter level. Furthermore, ND boosted the brain-derived neurotrophic factor (BDNF) and mitochondrial transcription factor-A (TFAM), suppressed the proinflammatory cytokine tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), and curbed oxidative stress by hampering of inducible nitric oxide synthase (iNOS). Moreover, ND augmented the hippocampal levels of phosphorylated signal transducer and activator of transcription-3 (p-STAT3) and B cell leukemia/lymphoma-2 (Bcl-2) anti-apoptotic protein while diminished nuclear factor-kappaB (NF-κB) and caspase-3 (casp-3) expression. These findings indicate the protective effect of ND against memory deficits and AD-like pathological aberrations probably via modulating NF-kB and STAT3 signaling, effects mediated likely by modulating N-methyl-D-aspartate (NMDA) receptors.

Aquaporin-4 and Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia. Although the pathogenesis of AD remains unclear, AD is thought to result from an imbalance in the production and clearance of amyloid-β protein (Aβ). Aquaporin-4 (AQP4) is the major aquaporin in the mammalian brain, is mostly expressed on astrocytic endfeet, and functions as a water transporter. However, the distribution and expression of AQP4 are altered in both AD clinical populations and animal models. Recent studies have revealed that AQP4 is important to the clearance of Aβ in brain via lymphatic clearance, transcytotic delivery, and glial degradation, as well as to the synaptic function. Thus, AQP4 likely plays an important role in the pathogenesis of AD. Further studies would provide new targets for prevention, ultimately leading to improved treatment options for AD.

In vivo Differential Brain Clearance and Catabolism of Monomeric and Oligomeric Alzheimer's Aβ protein

Amyloid β (Aβ) is the major constituent of the brain deposits found in parenchymal plaques and cerebral blood vessels of patients with Alzheimer's disease (AD). Several lines of investigation support the notion that synaptic pathology, one of the strongest correlates to cognitive impairment, is related to the progressive accumulation of neurotoxic Aβ oligomers. Since the process of oligomerization/fibrillization is concentration-dependent, it is highly reliant on the homeostatic mechanisms that regulate the steady state levels of Aβ influencing the delicate balance between rate of synthesis, dynamics of aggregation, and clearance kinetics. Emerging new data suggest that reduced Aβ clearance, particularly in the aging brain, plays a critical role in the process of amyloid formation and AD pathogenesis. Using well-defined monomeric and low molecular mass oligomeric Aβ1-40 species stereotaxically injected into the brain of C57BL/6 wild-type mice in combination with biochemical and mass spectrometric analyses in CSF, our data clearly demonstrate that Aβ physiologic removal is extremely fast and involves local proteolytic degradation leading to the generation of heterogeneous C-terminally cleaved proteolytic products, while providing clear indication of the detrimental role of oligomerization for brain Aβ efflux. Immunofluorescence confocal microscopy studies provide insight into the cellular pathways involved in the brain removal and cellular uptake of Aβ. The findings indicate that clearance from brain interstitial fluid follows local and systemic paths and that in addition to the blood-brain barrier, local enzymatic degradation and the bulk flow transport through the choroid plexus into the CSF play significant roles. Our studies highlight the diverse factors influencing brain clearance and the participation of various routes of elimination opening up new research opportunities for the understanding of altered mechanisms triggering AD pathology and for the potential design of combined therapeutic strategies.

Emerging technologies for protease engineering: New tools to clear out disease

Proteases regulate many biological processes through their ability to activate or inactive their target substrates. Because proteases catalytically turnover proteins and peptides, they present unique opportunities for use in biotechnological and therapeutic applications. However, many proteases are capable of cleaving multiple physiological substrates. Therefore their activity, expression, and localization are tightly controlled to prevent unwanted proteolysis. Currently, the use of protease therapeutics has been limited to a handful of proteases with narrow substrate specificities, which naturally limits their toxicity. Wider application of proteases is contingent upon the development of methods for engineering protease selectivity, activity, and stability. Recent advances in the development of high-throughput, bacterial and yeast-based methods for protease redesign have yielded protease variants with novel specificities, reduced toxicity, and increased resistance to inhibitors. Here, we highlight new tools for protease engineering, including methods suitable for the redesign of human secreted proteases, and future opportunities to exploit the catalytic activity of proteases for therapeutic benefit. Biotechnol. Bioeng. 2017;114: 33-38. © 2016 Wiley Periodicals, Inc.

Ablation of Prion Protein in Wild Type Human Amyloid Precursor Protein (APP) Transgenic Mice Does Not Alter The Proteolysis of APP, Levels of Amyloid-β or Pathologic Phenotype

The cellular prion protein (PrPC) has been proposed to play an important role in the pathogenesis of Alzheimer's disease. In cellular models PrPC inhibited the action of the β-secretase BACE1 on wild type amyloid precursor protein resulting in a reduction in amyloid-β (Aβ) peptides. Here we have assessed the effect of genetic ablation of PrPC in transgenic mice expressing human wild type amyloid precursor protein (line I5). Deletion of PrPC had no effect on the α- and β-secretase proteolysis of the amyloid precursor protein (APP) nor on the amount of Aβ38, Aβ40 or Aβ42 in the brains of the mice. In addition, ablation of PrPC did not alter Aβ deposition or histopathology phenotype in this transgenic model. Thus using this transgenic model we could not provide evidence to support the hypothesis that PrPC regulates Aβ production.

Design of Peptide Substrate for Sensitively and Specifically Detecting Two Aβ-Degrading Enzymes: Neprilysin and Angiotensin-Converting Enzyme

Upregulation of neprilysin (NEP) to reduce Aβ accumulation in the brain is a promising strategy for the prevention of Alzheimer's disease (AD). This report describes the design and synthesis of a quenched fluorogenic peptide substrate qf-Aβ(12-16)AAC (with the sequence VHHQKAAC), which has a fluorophore, Alexa-350, linked to the side-chain of its C-terminal cysteine and a quencher, Dabcyl, linked to its N-terminus. This peptide emitted strong fluorescence upon cleavage. Our results showed that qf-Aβ(12-16)AAC is more sensitive to NEP than the previously reported peptide substrates, so that concentrations of NEP as low as 0.03 nM could be detected at peptide concentration of 2 μM. Moreover, qf-Aβ(12-16)AAC had superior enzymatic specificity for both NEP and angiotensin-converting enzyme (ACE), but was inert with other Aβ-degrading enzymes. This peptide, used in conjunction with a previously reported peptide substrate qf-Aβ(1-7)C [which is sensitive to NEP and insulin-degrading enzyme (IDE)], could be used for high-throughput screening of compounds that only upregulate NEP. The experimental results of cell-based activity assays using both qf-Aβ(1-7)C and qf-Aβ(12-16)AAC as the substrates confirm that somatostatin treatment most likely upregulates IDE, but not NEP, in neuroblastoma cells.

Neuroprotective effects of ginseng protein on PI3K/Akt signaling pathway in the hippocampus of D-galactose/AlCl3 inducing rats model of Alzheimer's disease

ETHNOPHARMACOLOGICAL RELEVANCE

Alzheimer disease (AD) is a progressive neurodegenerative disease, with progressive memory loss, cognitive deterioration, and behavioral disorders. Ginseng (Panax ginseng C.A. Meyer) is widely used in China to treat various kinds of nervous system disorders. The study aimed to explore the therapeutic effect of ginseng protein (GP) on Alzheimer's disease and its correlation with the PI3K/Akt signaling pathway to understand the mechanism underlying the neuroprotective effect of ginseng.

MATERIAL AND METHODS

The AD rat model was established by intraperitoneally injecting D-galactose [60mg/(kgd)] followed by intragastrically administering AlCl3 [40mg/(kgd)] for 90 days. From day 60, the GP groups were intragastrically administered with GP 0.05 or 0.1g/kg twice daily for 30 days. The ethology of rats was tested by Morris water maze test. The content of Aβ1-42 and p-tau in the hippocampus of rats was detected by enzyme-linked immunosorbent assay. The expression of mRNAs and proteins of PI3K, Akt, phosphorylated Akt (p-Akt), Bcl-2, and Bax in the hippocampus was detected by real-time quantitative reverse transcription polymerase chain reaction and Western blot assay.

RESULTS

GP was found to significantly improve the memory ability of AD rats and prolong the times of crossing the platform and the percentage of residence time in the original platform quadrant of spatial probe test. GP also reduced the content of Aβ1-42 and p-tau and improved the mRNA and protein expression of PI3K, p-Akt/Akt, and Bcl-2/Bax in the hippocampus.

CONCLUSIONS

GP could improve the memory ability and reduce the content of Aβ1-42 and p-tau in AD rats. The anti-AD effects of GP were in part mediated by PI3K/Akt signaling pathway activation.

Amyloid cascade hypothesis: Pathogenesis and therapeutic strategies in Alzheimer's disease

Alzheimer's disease is an irreversible, progressive neurodegenerative disorder. Various therapeutic approaches are being used to improve the cholinergic neurotransmission, but their role in AD pathogenesis is still unknown. Although, an increase in tau protein concentration in CSF has been described in AD, but several issues remains unclear. Extensive and accurate analysis of CSF could be helpful to define presence of tau proteins in physiological conditions, or released during the progression of neurodegenerative disease. The amyloid cascade hypothesis postulates that the neurodegeneration in AD caused by abnormal accumulation of amyloid beta (Aβ) plaques in various areas of the brain. The amyloid hypothesis has continued to gain support over the last two decades, particularly from genetic studies. Therefore, current research progress in several areas of therapies shall provide an effective treatment to cure this devastating disease. This review critically evaluates general biochemical and physiological functions of Aβ directed therapeutics and their relevance.

Sequential Amyloid-β Degradation by the Matrix Metalloproteases MMP-2 and MMP-9

Matrix metalloproteases (MMPs) MMP-2 and MMP-9 have been implicated in the physiological catabolism of Alzheimer's amyloid-β (Aβ). Conversely, their association with vascular amyloid deposits, blood-brain barrier disruption, and hemorrhagic transformations after ischemic stroke also highlights their involvement in pathological processes. To better understand this dichotomy, recombinant human (rh) MMP-2 and MMP-9 were incubated with Aβ40 and Aβ42, and the resulting proteolytic fragments were assessed via immunoprecipitation and quantitative mass spectrometry. Both MMPs generated Aβ fragments truncated only at the C terminus, ending at positions 34, 30, and 16. Using deuterated homologues as internal standards, we observed limited and relatively slow degradation of Aβ42 by rhMMP-2, although the enzyme cleaved >80% of Aβ40 during the 1st h of incubation. rhMMP-9 was significantly less effective, particularly in degrading Aβ(1-42), although the targeted peptide bonds were identical. Using Aβ(1-34) and Aβ(1-30), we demonstrated that these peptides are also substrates for both MMPs, cleaving Aβ(1-34) to produce Aβ(1-30) first and Aβ(1-16) subsequently. Consistent with the kinetics observed with full-length Aβ, rhMMP-9 degraded only a minute fraction of Aβ(1-34) and was even less effective in producing Aβ(1-16). Further degradation of Aβ(1-16) by either MMP-2 or MMP-9 was not observed even after prolonged incubation times. Notably, all MMP-generated C-terminally truncated Aβ fragments were highly soluble and did not exhibit fibrillogenic properties or induce cytotoxicity in human cerebral microvascular endothelial or neuronal cells supporting the notion that these truncated Aβ species are associated with clearance mechanisms rather than being key elements in the fibrillogenesis process.

New therapeutic approaches for Alzheimer's disease and cerebral amyloid angiopathy

Accumulating evidence has shown a strong relationship between Alzheimer’s disease (AD), cerebral amyloid angiopathy (CAA), and cerebrovascular disease. Cognitive impairment in AD patients can result from cortical microinfarcts associated with CAA, as well as the synaptic and neuronal disturbances caused by cerebral accumulations of β-amyloid (Aβ) and tau proteins. The pathophysiology of AD may lead to a toxic chain of events consisting of Aβ overproduction, impaired Aβ clearance, and brain ischemia. Insufficient removal of Aβ leads to development of CAA and plays a crucial role in sporadic AD cases, implicating promotion of Aβ clearance as an important therapeutic strategy. Aβ is mainly eliminated by three mechanisms: (1) enzymatic/glial degradation, (2) transcytotic delivery, and (3) perivascular drainage (3-“d” mechanisms). Enzymatic degradation may be facilitated by activation of Aβ-degrading enzymes such as neprilysin, angiotensin-converting enzyme, and insulin-degrading enzyme. Transcytotic delivery can be promoted by inhibition of the receptor for advanced glycation end products (RAGE), which mediates transcytotic influx of circulating Aβ into brain. Successful use of the RAGE inhibitor TTP488 in Phase II testing has led to a Phase III clinical trial for AD patients. The perivascular drainage system seems to be driven by motive force generated by cerebral arterial pulsations, suggesting that vasoactive drugs can facilitate Aβ clearance. One of the drugs promoting this system is cilostazol, a selective inhibitor of type 3 phosphodiesterase. The clearance of fluorescent soluble Aβ tracers was significantly enhanced in cilostazol-treated CAA model mice. Given that the balance between Aβ synthesis and clearance determines brain Aβ accumulation, and that Aβ is cleared by several pathways stated above, multi-drugs combination therapy could provide a mainstream cure for sporadic AD.

A label-free electrical impedimetric biosensor for the specific detection of Alzheimer's amyloid-beta oligomers

Alzheimer's disease (AD) is the most common form of dementia, with over 37 million sufferers worldwide and a global cost of over $600 billion. There is currently no cure for AD and no reliable method of diagnosis other than post-mortem brain examination. The development of a point-of-care test for AD is an urgent requirement in order to provide earlier diagnosis and, thus, useful therapeutic intervention. Here, we present a novel, label-free impedimetric biosensor for the specific detection of amyloid-beta oligomers (AβO), which are the primary neurotoxic species in AD. AβO have been proposed as the best biomarker for AD and levels of AβO in the blood have been found to correlate with cerebrospinal fluid load. The biorecognition element of our biosensor is a fragment of the cellular prion protein (PrP(C), residues 95-110), a highly expressed synaptic protein which mediates the neuronal binding and toxicity of AβO. During the layer-by-layer sensor construction, biotinylated PrP(C) (95-110) was attached via a biotin/NeutrAvidin bridge to polymer-functionalised gold screen-printed electrodes. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry and scanning electron microscopy were used to validate biosensor assembly and functionality. EIS was employed for biosensor interrogation in the presence of Aβ oligomers or monomers. The biosensor was specific for the detection of synthetic AβO and gave a linear response, without significant detection of monomeric Aβ, down to an equivalent AβO concentration of ~0.5 pM. The biosensor was also able to detect natural, cell-derived AβO present in conditioned medium. The eventual commercialisation of this biosensor system could allow for the early diagnosis and disease monitoring of AD.

Carnosine's effect on amyloid fibril formation and induced cytotoxicity of lysozyme

Carnosine, a common dipeptide in mammals, has previously been shown to dissemble alpha-crystallin amyloid fibrils. To date, the dipeptide's anti-fibrillogensis effect has not been thoroughly characterized in other proteins. For a more complete understanding of carnosine's mechanism of action in amyloid fibril inhibition, we have investigated the effect of the dipeptide on lysozyme fibril formation and induced cytotoxicity in human neuroblastoma SH-SY5Y cells. Our study demonstrates a positive correlation between the concentration and inhibitory effect of carnosine against lysozyme fibril formation. Molecular docking results show carnosine's mechanism of fibrillogenesis inhibition may be initiated by binding with the aggregation-prone region of the protein. The dipeptide attenuates the amyloid fibril-induced cytotoxicity of human neuronal cells by reducing both apoptotic and necrotic cell deaths. Our study provides solid support for carnosine's amyloid fibril inhibitory property and its effect against fibril-induced cytotoxicity in SH-SY5Y cells. The additional insights gained herein may pave way to the discovery of other small molecules that may exert similar effects against amyloid fibril formation and its associated neurodegenerative diseases.

Contributions of degradation and brain-to-blood elimination across the blood-brain barrier to cerebral clearance of human amyloid-β peptide(1-40) in mouse brain

The purpose of the present study was to estimate the relative contributions of degradation and brain-to-blood elimination processes to the clearance of microinjected human amyloid-β peptide(1-40) (hAβ(1-40)) from mouse cerebral cortex, using a solid-phase extraction method together with a newly developed ultraperformance liquid chromatography/tandem mass spectrometry (UPLC/MS/MS) quantitation method for intact hAβ(1-40). The clearance rate constant of hAβ(1-40) in mouse cerebral cortex was determined to be 3.21 × 10(-2)/min under conditions where the saturable brain-to-blood elimination process across the blood-brain barrier (BBB) was expected to be saturated. Thus, this clearance rate constant should mainly reflect degradation. The [(125)I]hAβ(1-40) elimination rate across the BBB under nonsaturating conditions was determined to be 1.48 × 10(-2)/min. Inhibition studies suggested that processes sensitive to insulin and phosphoramidon, which inhibit neprilysin, insulin-degrading enzyme, and endothelin-converting enzyme, are involved not only in degradation, but also in elimination of hAβ(1-40). In conclusion, our results suggest a dominant contribution of degradation to cerebral hAβ(1-40) clearance, and also indicate that a sequential process of degradation and elimination of degradation products is involved in cerebral hAβ(1-40) clearance.