Loss of kallikrein-related peptidase 7 exacerbates amyloid pathology in Alzheimer's disease model mice

Involvement of the glymphatic/meningeal lymphatic system in Alzheimer's disease: insights into proteostasis and future directions

BACKGROUND

 Alzheimer's disease (AD) is pathologically characterized by the abnormal accumulation of Aβ and tau proteins. There has long been a keen interest among researchers in understanding how Aβ and tau are ultimately cleared in the brain. The discovery of this glymphatic system introduced a novel perspective on protein clearance and it gained recognition as one of the major brain clearance pathways for clearing these pathogenic proteins in AD. This finding has sparked interest in exploring the potential contribution of the glymphatic/meningeal lymphatic system in AD. Furthermore, there is a growing emphasis and discussion regarding the possibility that activating the glymphatic/meningeal lymphatic system could serve as a novel therapeutic strategy against AD.

OBJECTIVES

 Given this current research trend, the primary focus of this comprehensive review is to highlight the role of the glymphatic/meningeal lymphatic system in the pathogenesis of AD. The discussion will encompass future research directions and prospects for treatment in relation to the glymphatic/meningeal lymphatic system.

Kallikrein-related peptidase's significance in Alzheimer's disease pathogenesis: A comprehensive survey

Alzheimer's disease (AD) and related dementias constitute an important global health challenge. Detailed understanding of the multiple molecular mechanisms underlying their pathogenesis constitutes a clue for the management of the disease. Kallikrein-related peptidases (KLKs), a lead family of serine proteases, have emerged as potential biomarkers and therapeutic targets in the context of AD and associated cognitive decline. Hence, KLKs were proposed to display multifaceted impacts influencing various aspects of neurodegeneration, including amyloid-beta aggregation, tau pathology, neuroinflammation, and synaptic dysfunction. We propose here a comprehensive survey to summarize recent findings, providing an overview of the main kallikreins implicated in AD pathophysiology namely KLK8, KLK6 and KLK7. We explore the interplay between KLKs and key AD molecular pathways, shedding light on their significance as potential biomarkers for early disease detection. We also discuss their pertinence as therapeutic targets for disease-modifying interventions to develop innovative therapeutic strategies aimed at halting or ameliorating the progression of AD and associated dementias.

Protective effect of PDE4B subtype-specific inhibition in an App knock-in mouse model for Alzheimer's disease

Meta-analysis of genome-wide association study data has implicated PDE4B in the pathogenesis of Alzheimer's disease (AD), the leading cause of senile dementia. PDE4B encodes one of four subtypes of cyclic adenosine monophosphate (cAMP)-specific phosphodiesterase-4 (PDE4A-D). To interrogate the involvement of PDE4B in the manifestation of AD-related phenotypes, the effects of a hypomorphic mutation (Pde4bY358C) that decreases PDE4B's cAMP hydrolytic activity were evaluated in the AppNL-G-F knock-in mouse model of AD using the Barnes maze test of spatial memory, 14C-2-deoxyglucose autoradiography, thioflavin-S staining of β-amyloid (Aβ) plaques, and inflammatory marker assay and transcriptomic analysis (RNA sequencing) of cerebral cortical tissue. At 12 months of age, AppNL-G-F mice exhibited spatial memory and brain metabolism deficits, which were prevented by the hypomorphic PDE4B in AppNL-G-F/Pde4bY358C mice, without a decrease in Aβ plaque burden. RNA sequencing revealed that, among the 531 transcripts differentially expressed in AppNL-G-F versus wild-type mice, only 13 transcripts from four genes - Ide, Btaf1, Padi2, and C1qb - were differentially expressed in AppNL-G-F/Pde4bY358C versus AppNL-G-F mice, identifying their potential involvement in the protective effect of hypomorphic PDE4B. Our data demonstrate that spatial memory and cerebral glucose metabolism deficits exhibited by 12-month-old AppNL-G-F mice are prevented by targeted inhibition of PDE4B. To our knowledge, this is the first demonstration of a protective effect of PDE4B subtype-specific inhibition in a preclinical model of AD. It thus identifies PDE4B as a key regulator of disease manifestation in the AppNL-G-F model and a promising therapeutic target for AD.

Dimethyl fumarate improves cognitive impairment and neuroinflammation in mice with Alzheimer's disease

BACKGROUND

Neuroinflammation substantially contributes to the pathology of Alzheimer's disease (AD), the most common form of dementia. Studies have reported that nuclear factor erythroid 2-related factor 2 (Nrf2) attenuates neuroinflammation in the mouse models of neurodegenerative diseases, however, the detailed mechanism remains unclear.

METHODS

The effects of dimethyl fumarate (DMF), a clinically used drug to activate the Nrf2 pathway, on neuroinflammation were analyzed in primary astrocytes and AppNL-G-F (App-KI) mice. The cognitive function and behavior of DMF-administrated App-KI mice were evaluated. For the gene expression analysis, microglia and astrocytes were directly isolated from the mouse cerebral cortex by magnetic-activated cell sorting, followed by quantitative PCR.

RESULTS

DMF treatment activated some Nrf2 target genes and inhibited the expression of proinflammatory markers in primary astrocytes. Moreover, chronic oral administration of DMF attenuated neuroinflammation, particularly in astrocytes, and reversed cognitive dysfunction presumably by activating the Nrf2-dependent pathway in App-KI mice. Furthermore, DMF administration inhibited the expression of STAT3/C3 and C3 receptor in astrocytes and microglia isolated from App-KI mice, respectively, suggesting that the astrocyte-microglia crosstalk is involved in neuroinflammation in mice with AD.

CONCLUSION

The activation of astrocytic Nrf2 signaling confers neuroprotection in mice with AD by controlling neuroinflammation, particularly by regulating astrocytic C3-STAT3 signaling. Furthermore, our study has implications for the repositioning of DMF as a drug for AD treatment.

Microglia and Astrocytes in Alzheimer's Disease: Significance and Summary of Recent Advances

Alzheimer's disease, one of the most common forms of dementia, is characterized by a slow progression of cognitive impairment and neuronal loss. Currently, approved treatments for AD are hindered by various side effects and limited efficacy. Despite considerable research, practical treatments for AD have not been developed. Increasing evidence shows that glial cells, especially microglia and astrocytes, are essential in the initiation and progression of AD. During AD progression, activated resident microglia increases the ability of resting astrocytes to transform into reactive astrocytes, promoting neurodegeneration. Extensive clinical and molecular studies show the involvement of microglia and astrocyte-mediated neuroinflammation in AD pathology, indicating that microglia and astrocytes may be potential therapeutic targets for AD. This review will summarize the significant and recent advances of microglia and astrocytes in the pathogenesis of AD in three parts. First, we will review the typical pathological changes of AD and discuss microglia and astrocytes in terms of function and phenotypic changes. Second, we will describe microglia and astrocytes' physiological and pathological role in AD. These roles include the inflammatory response, "eat me" and "don't eat me" signals, Aβ seeding, propagation, clearance, synapse loss, synaptic pruning, remyelination, and demyelination. Last, we will review the pharmacological and non-pharmacological therapies targeting microglia and astrocytes in AD. We conclude that microglia and astrocytes are essential in the initiation and development of AD. Therefore, understanding the new role of microglia and astrocytes in AD progression is critical for future AD studies and clinical trials. Moreover, pharmacological, and non-pharmacological therapies targeting microglia and astrocytes, with specific studies investigating microglia and astrocyte-mediated neuronal damage and repair, may be a promising research direction for future studies regarding AD treatment and prevention.

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).

FP-MAP: an extensive library of fingerprint-based molecular activity prediction tools

Discovering new drugs for disease treatment is challenging, requiring a multidisciplinary effort as well as time, and resources. With a view to improving hit discovery and lead compound identification, machine learning (ML) approaches are being increasingly used in the decision-making process. Although a number of ML-based studies have been published, most studies only report fragments of the wider range of bioactivities wherein each model typically focuses on a particular disease. This study introduces FP-MAP, an extensive atlas of fingerprint-based prediction models that covers a diverse range of activities including neglected tropical diseases (caused by viral, bacterial and parasitic pathogens) as well as other targets implicated in diseases such as Alzheimer's. To arrive at the best predictive models, performance of ≈4,000 classification/regression models were evaluated on different bioactivity data sets using 12 different molecular fingerprints. The best performing models that achieved test set AUC values of 0.62-0.99 have been integrated into an easy-to-use graphical user interface that can be downloaded from https://gitlab.com/vishsoft/fpmap.

A Metalloproteinase Cocktail from the Venom of Protobothrops flavoviridis Cleaves Amyloid Beta Peptides at the α-Cleavage Site

A disintegrin and metalloproteinase (ADAM) family proteins are a major class of membrane-anchored multidomain proteinases that are responsible for the shedding of cell surface protein ectodomains, including amyloid precursor protein (APP). Human ADAM 9, 10, and 17 proteolyze APPs and produce non-amyloid-genic p3 peptides, instead of neurotoxic amyloid-β peptides (Aβs; Aβ40 and Aβ42), which form fibrils and accumulate in the brain of patients with Alzheimer's disease (AD). The ADAM family is closely related to snake venom metalloproteinases (SVMPs), which are derived from ancestral ADAMs but act as soluble proteinases. To test the therapeutic potential of SVMPs, we purified SVMPs from Protobothrops flavoviridis venom using metal ion affinity and pooled into a cocktail. Thus, 9 out of 11 SVMPs in the P. flavoviridis genome were identified in the cocktail. SVMPs inhibited Aβ secretion when added to human cell culture medium without affecting APP proteolysis. SVMPs degraded synthetic Aβ40 and Aβ42 peptides at the same cleavage site (α-site of APP) as ADAM9, 10, and 17. SVMPs did not degrade Aβ fibrils but interfered with their formation, assessed using thioflavin-T. Thus, SVMPs have therapeutic potential for AD as an Aβ-degrading protease, and the finding adds to the discovery of bioactive peptides from venoms as novel therapeutics.

A blood-based composite panel that screens Alzheimer's disease

BACKGROUND

Blood tests would be much easier to implement in the clinical diagnosis of Alzheimer's disease (AD) as minimally invasive measurements. Multiple inspection technologies promoted AD-associated blood biomarkers' exploration. However, there was a lack of further screening and validation for these explored blood-based biomarkers. We selected four potential biomarkers to explore their plasma levels in AD and amnestic mild cognitive impairment (aMCI) and developed a composite panel for AD and aMCI screening.

METHOD

The plasma concentrations of soluble low-density lipoprotein receptor-associated protein 1 (sLRP1), Gelsolin (GSN), Kallikrein 4 (KLK4) and Caspase 3 were measured in the discovery and validation cohort. The receiver operating characteristic (ROC) curve was generated to assess the classification panel with the area under the curve (AUC).

RESULTS

A total of 233 participants (26 CN, 27 aMCI, and 26 AD in the discovery cohort, and 51 CN, 50 aMCI, and 53 AD in the validation cohort) with complete data were included in the study. The plasma concentrations of sLRP1 and Caspase 3 were significantly decreased in AD and aMCI when compared with those in the CN group. Compared with the CN group, the concentrations of KLK4 and GSN were increased in AD, but not in MCI. Interestingly, one of four proteins, sLRP1 in plasma level was higher in Apolipoprotein E (APOE) ε4 non-carriers than that in APOE ε4 carriers, especially among CN and MCI. No significant difference was found between females and males in the plasma levels of four proteins. The composite panel is based on four blood biomarkers accurately classifying AD from CN (AUC = 0.903-0.928), and MCI from CN (AUC = 0.846-0.865). Moreover, dynamic changes in the plasma levels of four proteins exhibited a significant correlation with cognitive assessment.

CONCLUSIONS

Altogether, these findings indicate that the plasma levels of sLRP1, KLK4, GSN and Caspase 3 changed with the progression of AD. And their combination could be used to develop a panel for classifying AD and aMCI with high accuracy, which would provide an alternative approach for developing a blood-based test for AD and aMCI screening.

INPP5D modulates TREM2 loss-of-function phenotypes in a β-amyloidosis mouse model

The genetic associations of TREM2 loss-of-function variants with Alzheimer disease (AD) indicate the protective roles of microglia in AD pathogenesis. Functional deficiencies of TREM2 disrupt microglial clustering around amyloid β (Aβ) plaques, impair their transcriptional response to Aβ, and worsen neuritic dystrophy. However, the molecular mechanism underlying these phenotypes remains unclear. In this study, we investigated the pathological role of another AD risk gene, INPP5D, encoding a phosphoinositide PI(3,4,5)P3 phosphatase expressed in microglia. In a Tyrobp-deficient TREM2 loss-of-function mouse model, Inpp5d haplodeficiency restored the association of microglia with Aβ plaques, partially restored plaque compaction, and astrogliosis, and reduced phosphorylated tau+ dystrophic neurites. Mechanistic analyses suggest that TREM2/TYROBP and INPP5D exert opposing effects on PI(3,4,5)P3 signaling pathways as well as on phosphoproteins involved in the actin assembly. Our results suggest that INPP5D acts downstream of TREM2/TYROBP to regulate the microglial barrier against Aβ toxicity, thereby modulates Aβ-dependent pathological conversion of tau.

The original strain of SARS-CoV-2, the Delta variant, and the Omicron variant infect microglia efficiently, in contrast to their inability to infect neurons: Analysis using 2D and 3D cultures

COVID-19 causes neurological damage, systemic inflammation, and immune cell abnormalities. COVID-19-induced neurological impairment may be caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which directly infects cells of the central nervous system (CNS) and exerts toxic effects. Furthermore, SARS-CoV-2 mutations occur constantly, and it is not well understood how the infectivity of the virus to cells of the CNS changes as the virus mutates. Few studies have examined whether the infectivity of cells of CNS - neural stem/progenitor cells (NS/PCs), neurons, astrocytes, and microglia - varies among SARS-CoV-2 mutant strains. In this study, therefore, we investigated whether SARS-CoV-2 mutations increase infectivity to CNS cells, including microglia. Since it was essential to demonstrate the infectivity of the virus to CNS cells in vitro using human cells, we generated cortical neurons, astrocytes, and microglia from human induced pluripotent stem cells (hiPSCs). We added pseudotyped lentiviruses of SARS-CoV-2 to each type of cells, and then we examined their infectivity. We prepared three pseudotyped lentiviruses expressing the S protein of the original strain (the first SARS-CoV-2 discovered in the world), the Delta variant, and the Omicron variant on their envelopes and analyzed differences of their ability to infect CNS cells. We also generated brain organoids and investigated the infectivity of each virus. The viruses did not infect cortical neurons, astrocytes, or NS/PCs, but microglia were infected by the original, Delta, and Omicron pseudotyped viruses. In addition, DPP4 and CD147, potential core receptors of SARS-CoV-2, were highly expressed in the infected microglia, while DPP4 expression was deficient in cortical neurons, astrocytes, and NS/PCs. Our results suggest that DPP4, which is also a receptor for Middle East respiratory syndrome-coronavirus (MERS-CoV), may play an essential role in the CNS. Our study is applicable to the validation of the infectivity of viruses that cause various infectious diseases in CNS cells, which are difficult to sample from humans.

4-Methylumbelliferone Targets Revealed by Public Data Analysis and Liver Transcriptome Sequencing

4-methylumbelliferone (4MU) is a well-known hyaluronic acid synthesis inhibitor and an approved drug for the treatment of cholestasis. In animal models, 4MU decreases inflammation, reduces fibrosis, and lowers body weight, serum cholesterol, and insulin resistance. It also inhibits tumor progression and metastasis. The broad spectrum of effects suggests multiple and yet unknown targets of 4MU. Aiming at 4MU target deconvolution, we have analyzed publicly available data bases, including: 1. Small molecule library Bio Assay screening (PubChemBioAssay); 2. GO pathway databases screening; 3. Protein Atlas Database. We also performed comparative liver transcriptome analysis of mice on normal diet and mice fed with 4MU for two weeks. Potential targets of 4MU public data base analysis fall into two big groups, enzymes and transcription factors (TFs), including 13 members of the nuclear receptor superfamily regulating lipid and carbohydrate metabolism. Transcriptome analysis revealed changes in the expression of genes involved in bile acid metabolism, gluconeogenesis, and immune response. It was found that 4MU feeding decreased the accumulation of the glycogen granules in the liver. Thus, 4MU has multiple targets and can regulate cell metabolism by modulating signaling via nuclear receptors.

Poly(I:C) promotes neurotoxic amyloid β accumulation through reduced degradation by decreasing neprilysin protein levels in astrocytes

Inflammation associated with viral infection of the nervous system has been involved in the pathogenesis of neurodegenerative diseases, such as Alzheimer's disease (AD) and multiple sclerosis. Polyinosinic:polycytidylic acid (poly[I:C]) is a Toll-like receptor 3 (TLR3) agonist that mimics the inflammatory response to systemic viral infections. Despite growing recognition of the role of glial cells in AD pathology, their involvement in the accumulation and clearance of amyloid β (Aβ) in the brain of patients with AD is poorly understood. Neprilysin (NEP) and insulin-degrading enzyme (IDE) are the main Aβ-degrading enzymes in the brain. This study investigated whether poly(I:C) regulated Aβ degradation and neurotoxicity by modulating NEP and IDE protein levels through TLR3 in astrocytes. To this aim, primary rat primary astrocyte cultures were treated with poly(I:C) and inhibitors of the TLR3 signaling. Protein levels were assessed by Western blot. Aβ toxicity to primary neurons was measured by lactate dehydrogenase release. Poly(I:C) induced a significant decrease in NEP levels on the membrane of astrocytes as well as in the culture medium. The degradation of exogenous Aβ was markedly delayed in poly(I:C)-treated astrocytes. This delay significantly increased the neurotoxicity of exogenous Aβ1-42. Altogether, these results suggest that viral infections induce Aβ neurotoxicity by decreasing NEP levels in astrocytes and consequently preventing Aβ degradation.

Comparative studies for amyloid beta degradation: “Neprilysin vs insulysin”, “monomeric vs aggregate”, and “whole Aβ40 vs its peptide fragments”

Amyloid beta (Aβ) proteins are produced from amyloid precursor protein cleaved by β- and γ-secretases, and are the main components of senile plaques pathologically found in Alzheimer's disease (AD) patient brains. Therefore, the relationship between AD and Aβs has been well studied for both therapeutic and diagnostic purposes. Several enzymes have been reported to degrade Aβs in vivo, with neprilysin (NEP) and insulysin (insulin-degrading enzyme, IDE) being the most prominent. In this article, we describe the mass spectrometric characterization of peptide fragments generated using NEP and IDE, and clarify the differences in digestion specificities between these two enzymes for non-aggregated Aβ₄₀, aggregated Aβ₄₀, and Aβ₄₀ peptide fragments, including Aβ₁₆. Our results allowed identification of all the peptide fragments from non-aggregated Aβ₄₀: NEP, 23 peptide fragments consisting of 2–11 amino-acid residues, 17 cleavage sites; IDE, 23 peptide fragments consisting of 6–33 amino-acid residues, 15 cleavage sites. Also, we confirmed that IDE can digest only whole Aβ₄₀, whereas NEP can digest both Aβ₄₀ and partial structures such as Aβ₁₆ and peptide fragments generated by the digestion of Aβ₄₀ by IDE. Furthermore, we confirmed that IDE and NEP are unable to digest aggregated Aβ₄₀.

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Two Aβ degrading enzymes (neprilysin, NEP and insulysin, IDE) were examined.

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NEP digested Aβ₄₀ into 23 peptides consisting of 2–11 amino-acid residues.

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IDE digested Aβ₄₀ into 23 peptides consisting of 6–33 amino-acid residues.

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Neither IDE nor NEP can digest aggregated Aβ₄₀.

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NEP can digest Aβ fragment peptides including Aβ₁₆, although IDE cannot.

Proteomics and functional study reveal kallikrein-6 enhances communicating hydrocephalus

Background

Communicating hydrocephalus (CH) is a common neurological disorder caused by a blockage of cerebrospinal fluid. In this study, we aimed to explore the potential molecular mechanism underlying CH development.

Methods

Quantitative proteomic analysis was performed to screen the differentially expressed proteins (DEPs) between patients with and without CH. A CH rat model was verified by Hoechst staining, and the co-localization of the target protein and neuron was detected using immunofluorescence staining. Loss-of-function experiments were performed to examine the effect of KLK6 on the synapse structure.

Results

A total of 11 DEPs were identified, and kallikrein 6 (KLK6) expression was found to be significantly upregulated in patients with CH compared with that in patients without CH. The CH rat model was successfully constructed, and KLK6 was found to be co-localized with neuronal nuclei in brain tissue. The expression level of IL-1β, TNF-α, and KLK6 in the CH group was higher than that in the control group. After knockdown of KLK6 expression using small-interfering RNA (siRNA), the expression levels of synapsin-1 and PSD95 in neuronal cells were increased, and the length, number, and structure of synapses were significantly improved. Following siRNA interference KLK6 expression, 5681 differentially expressed genes (DEGs) were identified in transcriptome profile. The upregulated DEGs of Appl2, Nav2, and Nrn1 may be involved in the recovery of synaptic structures after the interference of KLK6 expression.

Conclusions

Collectively, KLK6 participates in the development of CH and might provide a new target for CH treatment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12014-021-09335-9.

Successful Treatment of Noise-Induced Hearing Loss by Mesenchymal Stromal Cells: An RNAseq Analysis of Protective/Repair Pathways

Mesenchymal stromal cells (MSCs) are an adult derived stem cell-like population that has been shown to mediate repair in a wide range of degenerative disorders. The protective effects of MSCs are mainly mediated by the release of growth factors and cytokines thereby modulating the diseased environment and the immune system. Within the inner ear, MSCs have been shown protective against tissue damage induced by sound and a variety of ototoxins. To better understand the mechanism of action of MSCs in the inner ear, mice were exposed to narrow band noise. After exposure, MSCs derived from human umbilical cord Wharton's jelly were injected into the perilymph. Controls consisted of mice exposed to sound trauma only. Forty-eight hours post-cell delivery, total RNA was extracted from the cochlea and RNAseq performed to evaluate the gene expression induced by the cell therapy. Changes in gene expression were grouped together based on gene ontology classification. A separate cohort of animals was treated in a similar fashion and allowed to survive for 2 weeks post-cell therapy and hearing outcomes determined. Treatment with MSCs after severe sound trauma induced a moderate hearing protective effect. MSC treatment resulted in an up-regulation of genes related to immune modulation, hypoxia response, mitochondrial function and regulation of apoptosis. There was a down-regulation of genes related to synaptic remodeling, calcium homeostasis and the extracellular matrix. Application of MSCs may provide a novel approach to treating sound trauma induced hearing loss and may aid in the identification of novel strategies to protect hearing.

The Amyloid-β Pathway in Alzheimer's Disease

Breakthroughs in molecular medicine have positioned the amyloid-β (Aβ) pathway at the center of Alzheimer's disease (AD) pathophysiology. While the detailed molecular mechanisms of the pathway and the spatial-temporal dynamics leading to synaptic failure, neurodegeneration, and clinical onset are still under intense investigation, the established biochemical alterations of the Aβ cycle remain the core biological hallmark of AD and are promising targets for the development of disease-modifying therapies. Here, we systematically review and update the vast state-of-the-art literature of Aβ science with evidence from basic research studies to human genetic and multi-modal biomarker investigations, which supports a crucial role of Aβ pathway dyshomeostasis in AD pathophysiological dynamics. We discuss the evidence highlighting a differentiated interaction of distinct Aβ species with other AD-related biological mechanisms, such as tau-mediated, neuroimmune and inflammatory changes, as well as a neurochemical imbalance. Through the lens of the latest development of multimodal in vivo biomarkers of AD, this cross-disciplinary review examines the compelling hypothesis- and data-driven rationale for Aβ-targeting therapeutic strategies in development for the early treatment of AD.

Suppression of amyloid-β secretion from neurons by cis-9, trans-11-octadecadienoic acid, an isomer of conjugated linoleic acid

Two common conjugated linoleic acids (LAs), cis-9, trans-11 CLA (c9,t11 CLA) and trans-10, cis-12 CLA (t10,c12 CLA), exert various biological activities. However, the effect of CLA on the generation of neurotoxic amyloid-β (Aβ) protein remains unclear. We found that c9,t11 CLA significantly suppressed the generation of Aβ in mouse neurons. CLA treatment did not affect the level of β-site APP-cleaving enzyme 1 (BACE1), a component of active γ-secretase complex presenilin 1 amino-terminal fragment, or Aβ protein precursor (APP) in cultured neurons. BACE1 and γ-secretase activities were not directly affected by c9,t11 CLA. Localization of BACE1 and APP in early endosomes increased in neurons treated with c9,t11 CLA; concomitantly, the localization of both proteins was reduced in late endosomes, the predominant site of APP cleavage by BACE1. The level of CLA-containing phosphatidylcholine (CLA-PC) increased dramatically in neurons incubated with CLA. Incorporation of phospholipids containing c9,t11 CLA, but not t10,c12 CLA, into the membrane may affect the localization of some membrane-associated proteins in intracellular membrane compartments. Thus, in neurons treated with c9,t11 CLA, reduced colocalization of APP with BACE1 in late endosomes may decrease APP cleavage by BACE1 and subsequent Aβ generation. Our findings suggest that accumulation of c9,t11 CLA-PC/LPC in neuronal membranes suppresses production of neurotoxic Aβ in neurons.

Generation of a quenched phosphonate activity-based probe for labelling the active KLK7 protease

Kallikrein 7 (KLK7) is a chymotrypsin-like serine protease with established roles in skin diseases like the rare Netherton syndrome, an overdesquamating and inflammatory condition, but also common atopic dermatitis, and a potential drug target for these and possibly other diseases. Nevertheless, tools to determine the active KLK7 enzyme are not available. Here, a mixed alkyl aryl phosphonate quenched activity-based probe that detects the active KLK7 was developed and evaluated in vitro. This KLK7-qABP can potentially be used to monitor KLK7 activity in vivo.

GPR120 Signaling Controls Amyloid-β Degrading Activity of Matrix Metalloproteinases

Alzheimer's disease (AD) is characterized by the extensive deposition of amyloid-β peptide (Aβ) in the brain. Brain Aβ level is regulated by a balance between Aβ production and clearance. The clearance rate of Aβ is decreased in the brains of sporadic AD patients, indicating that the dysregulation of Aβ clearance mechanisms affects the pathologic process of AD. Astrocytes are among the most abundant cells in the brain and are implicated in the clearance of brain Aβ via their regulation of the blood-brain barrier, glymphatic system, and proteolytic degradation. The cellular morphology and activity of astrocytes are modulated by several molecules, including ω3 polyunsaturated fatty acids, such as docosahexaenoic acid, which is one of the most abundant lipids in the brain, via the G protein-coupled receptor GPR120/FFAR4. In this study, we analyzed the role of GPR120 signaling in the Aβ-degrading activity of astrocytes. Treatment with the selective antagonist upregulated the matrix metalloproteinase (MMP) inhibitor-sensitive Aβ-degrading activity in primary astrocytes. Moreover, the inhibition of GPR120 signaling increased the levels of Mmp2 and Mmp14 mRNAs, and decreased the expression levels of tissue inhibitor of metalloproteinases 3 (Timp3) and Timp4, suggesting that GPR120 negatively regulates the astrocyte-derived MMP network. Finally, the intracerebral injection of GPR120-specific antagonist substantially decreased the levels of TBS-soluble Aβ in male AD model mice, and this effect was canceled by the coinjection of an MMP inhibitor. These data indicate that astrocytic GPR120 signaling negatively regulates the Aβ-degrading activity of MMPs.SIGNIFICANCE STATEMENT The level of amyloid β (Aβ) in the brain is a crucial determinant of the development of Alzheimer's disease. Here we found that astrocytes, which are the most abundant cell type in the CNS, harbor degrading activity against Aβ, which is regulated by GPR120 signaling. GPR120 is involved in the inflammatory response and obesity in peripheral organs. However, the pathophysiological role of GPR120 in Alzheimer's disease remains unknown. We found that selective inhibition of GPR120 signaling in astrocytes increased the Aβ-degrading activity of matrix metalloproteases. Our results suggest that GPR120 in astrocytes is a novel therapeutic target for the development of anti-Aβ therapeutics.

Dietary cis-9, trans-11-conjugated linoleic acid reduces amyloid β-protein accumulation and upregulates anti-inflammatory cytokines in an Alzheimer's disease mouse model

Conjugated linoleic acid (CLA) is an isomer of linoleic acid (LA). The predominant dietary CLA is cis-9, trans-11-CLA (c-9, t-11-CLA), which constitutes up to ~ 90% of total CLA and is thought to be responsible for the positive health benefits associated with CLA. However, the effects of c-9, t-11-CLA on Alzheimer's disease (AD) remain to be elucidated. In this study, we investigated the effect of dietary intake of c-9, t-11-CLA on the pathogenesis of an AD mouse model. We found that c-9, t-11-CLA diet-fed AD model mice significantly exhibited (1) a decrease in amyloid-β protein (Aβ) levels in the hippocampus, (2) an increase in the number of microglia, and (3) an increase in the number of astrocytes expressing the anti-inflammatory cytokines, interleukin-10 and 19 (IL-10, IL-19), with no change in the total number of astrocytes. In addition, liquid chromatography-tandem mass spectrometry (LC-MS/MS) and gas chromatographic analysis revealed that the levels of lysophosphatidylcholine (LPC) containing c-9, t-11-CLA (CLA-LPC) and free c-9, t-11-CLA were significantly increased in the brain of c-9, t-11-CLA diet-fed mice. Thus, dietary c-9, t-11-CLA entered the brain and appeared to exhibit beneficial effects on AD, including a decrease in Aβ levels and suppression of inflammation.

The Role of Cathepsin B in the Degradation of Aβ and in the Production of Aβ Peptides Starting With Ala2 in Cultured Astrocytes

Astrocytes may not only be involved in the clearance of Amyloid beta peptides (Aβ) in Alzheimer's disease (AD), but appear to produce N-terminally truncated Aβ (Aβ_n−x) independently of BACE1, which generates the N-Terminus of Aβ starting with Asp1 (Aβ_1−x). A candidate protease for the generation of Aβ_n−x is cathepsin B (CatB), especially since CatB has also been reported to degrade Aβ, which could explain the opposite roles of astrocytes in AD. In this study, we investigated the influence of CatB inhibitors and the deletion of the gene encoding CatB (CTSB) using CRISPR/Cas9 technology on Aβ_2−x and Aβ_1−x levels in cell culture supernatants by one- and two-dimensional Urea-SDS-PAGE followed by immunoblot. While the cell-permeant inhibitors E64d and CA-074 Me did not significantly affect the Aβ_1−x levels in supernatants of cultured chicken and human astrocytes, they did reduce the Aβ_2−x levels. In the glioma-derived cell line H4, the Aβ_2−x levels were likewise decreased in supernatants by treatment with the more specific, but cell-impermeant CatB-inhibitor CA-074, by CA-074 Me treatment, and by CTSB gene deletion. Additionally, a more than 2-fold increase in secreted Aβ_1−x was observed under the latter two conditions. The CA-074 Me-mediated increase of Aβ_1−x, but not the decrease of Aβ_2−x, was influenced by concomitant treatment with the vacuolar H⁺-ATPase inhibitor Bafilomycin A1. This indicated that non-lysosomal CatB mediated the production of Aβ_2−x in astrocytes, while the degradation of Aβ_1−x seemed to be dependent on lysosomal CatB in H4 cells, but not in primary astrocytes. These findings highlight the importance of considering organelle targeting in drug development to promote Aβ degradation.

Modulation of β-Amyloid Fibril Formation in Alzheimer's Disease by Microglia and Infection

Amyloid plaques are a pathological hallmark of Alzheimer's disease. The major component of these plaques are highly ordered amyloid fibrils formed by amyloid-β (Aβ) peptides. However, whilst Aβ amyloid fibril assembly has been subjected to detailed and extensive analysis in vitro, these studies may not reproduce how Aβ fibrils assemble in the brain. This is because the brain represents a highly complex and dynamic environment, and in Alzheimer's disease multiple cofactors may affect the assembly of Aβ fibrils. Moreover, in vivo amyloid plaque formation will reflect the balance between the assembly of Aβ fibrils and their degradation. This review explores the roles of microglia as cofactors in Aβ aggregation and in the clearance of amyloid deposits. In addition, we discuss how infection may be an additional cofactor in Aβ fibril assembly by virtue of the antimicrobial properties of Aβ peptides. Crucially, by understanding the roles of microglia and infection in Aβ amyloid fibril assembly it may be possible to identify new therapeutic targets for Alzheimer's disease.

Deficiency of the serine peptidase Kallikrein 6 does not affect the levels and the pathological accumulation of a-synuclein in mouse brain

Several lines of evidence indicate that the propagation of misfolded α-synuclein (α-syn) plays a central role in the progression and manifestation of Parkinson's disease. Pathogenic α-syn species can be present in the extracellular space. Thus, the identification and modulation of the key enzymes implicated in extracellular α-syn turnover becomes vital. Kallikrein peptidase 6 has been identified as one of the major α-syn degrading enzymes and has been implicated in the clearance of extracellular α-syn. However, the physiological role of this enzyme in regulating α-syn, in vivo, still remains elusive. Here, by utilizing Klk6 knock-out (Klk6^-/- ) mice as our experimental model, we provide insight into the physiologic relevance of endogenous KLK6 expression on α-syn processing. Behavioral phenotyping showed that Klk6^-/- mice display no gross behavioral abnormalities. Further in vivo characterization of this mouse model, in the context of α-syn accumulation, showed that KLK6 deletion had no impact on the protein levels of intracellular or extracellular α-syn. Upon in vivo administration of α-syn pre-formed fibrils (PFF), α-syn pathologic accumulations were evident both in the brains of Klk6^-/- mice and wt mice without significant differences. Intrastriatal delivery of active KLK6, did not affect secreted α-syn levels observed in the A53T α-syn over-expressing mice. These findings suggest that in the in vivo setting of PFF pathology induction, KLK6 alone is not able to modulate pathology transmission. Our study raises implications for the use of recombinant α-syn fibrils in α-syn turnover studies.

Involvement of Kallikrein-Related Peptidases in Nervous System Disorders

Kallikrein-related peptidases (KLKs) are a family of serine proteases that when dysregulated may contribute to neuroinflammation and neurodegeneration. In the present review article, we describe what is known about their physiological and pathological roles with an emphasis on KLK6 and KLK8, two KLKs that are highly expressed in the adult central nervous system (CNS). Altered expression and activity of KLK6 have been linked to brain physiology and the development of multiple sclerosis. On the other hand, altered levels of KLK6 in the brain and serum of people affected by Alzheimer's disease and Parkinson's disease have been documented, pointing out to its function in amyloid metabolism and development of synucleinopathies. People who have structural genetic variants of KLK8 can suffer mental illnesses such as intellectual and learning disabilities, seizures, and autism. Increased expression of KLK8 has also been implicated in schizophrenia, bipolar disorder, and depression. Also, we discuss the possible link that exists between KLKs activity and certain viral infections that can affect the nervous system. Although little is known about the exact mechanisms that mediate KLKs function and their participation in neuroinflammatory and neurodegenerative disorders will open a new field to develop novel therapies to modulate their levels and/or activity and their harmful effects on the CNS.

MITOL deletion in the brain impairs mitochondrial structure and ER tethering leading to oxidative stress

MITOL deletion in mouse brain impairs the morphology and ER tethering of mitochondria, resulting in enhanced oxidative stress. This study suggests a relationship between morphological abnormalities of mitochondria and developmental disorder.

Mitochondrial abnormalities are associated with developmental disorders, although a causal relationship remains largely unknown. Here, we report that increased oxidative stress in neurons by deletion of mitochondrial ubiquitin ligase MITOL causes a potential neuroinflammation including aberrant astrogliosis and microglial activation, indicating that mitochondrial abnormalities might confer a risk for inflammatory diseases in brain such as psychiatric disorders. A role of MITOL in both mitochondrial dynamics and ER-mitochondria tethering prompted us to characterize three-dimensional structures of mitochondria in vivo. In MITOL-deficient neurons, we observed a significant reduction in the ER-mitochondria contact sites, which might lead to perturbation of phospholipids transfer, consequently reduce cardiolipin biogenesis. We also found that branched large mitochondria disappeared by deletion of MITOL. These morphological abnormalities of mitochondria resulted in enhanced oxidative stress in brain, which led to astrogliosis and microglial activation partly causing abnormal behavior. In conclusion, the reduced ER-mitochondria tethering and excessive mitochondrial fission may trigger neuroinflammation through oxidative stress.

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.

Astrocytes infected with Chlamydia pneumoniae demonstrate altered expression and activity of secretases involved in the generation of β-amyloid found in Alzheimer disease

Background

Epidemiologic studies strongly suggest that the pathophysiology of late-onset Alzheimer disease (AD) versus early-onset AD has environmental rather than genetic causes, thus revealing potentially novel therapeutic targets to limit disease progression. Several studies supporting the “pathogen hypothesis” of AD demonstrate a strong association between pathogens and the production of β-amyloid, the pathologic hallmark of AD. Although the mechanism of pathogen-induced neurodegeneration of AD remains unclear, astrocytes, a key player of the CNS innate immune response and producer/metabolizer of β-amyloid, have been implicated. We hypothesized that Chlamydia pneumoniae infection of human astrocytes alters the expression of the amyloid precursor protein (APP)-processing secretases, ADAM10, BACE1, and PSEN1, to promote β-amyloid formation. Utilizing immunofluorescent microscopy, molecular, and biochemical approaches, these studies explore the role of an intracellular respiratory pathogen, Chlamydia pneumoniae, as an environmental trigger for AD pathology. Human astrocytoma cells in vitro were infected with Chlamydia pneumoniae over the course of 6–72 h. The gene and protein expression, as well as the enzymatic activity of non-amyloidogenic (ADAM10), and pro-amyloidogenic (BACE1 and PSEN1) secretases were qualitatively and quantitatively assessed. In addition, the formation of toxic amyloid products as an outcome of pro-amyloidogenic APP processing was evaluated through various modalities.

Results

Chlamydia pneumoniae infection of human astrocytoma cells promoted the transcriptional upregulation of numerous genes implicated in host neuroinflammation, lipid homeostasis, microtubule function, and APP processing. Relative to that of uninfected astrocytes, BACE1 and PSEN1 protein levels were enhanced by nearly twofold at 48–72 h post-Chlamydia pneumoniae infection. The processing of APP in Chlamydia pneumoniae-infected astrocytes favors the pro-amyloidogenic pathway, as demonstrated by an increase in enzymatic activity of BACE1, while that of ADAM10 was decreased. Fluorescence intensity of β-amyloid and ELISA-quantified levels of soluble-APP by products revealed temporally similar increases, confirming a BACE1/PSEN1-mediated processing of APP.

Conclusions

Our findings suggest that Chlamydia pneumoniae infection of human astrocytes promotes the pro-amyloidogenic pathway of APP processing through the upregulation of expression and activity of β-secretase, upregulated expression of γ-secretase, and decreased activity of α-secretase. These effects of astrocyte infection provide evidence for a direct link between Chlamydia pneumoniae and AD pathology.

Electronic supplementary material

The online version of this article (10.1186/s12868-019-0489-5) contains supplementary material, which is available to authorized users.

Neuroinflammation in mouse models of Alzheimer's disease

Alzheimer's disease (AD) is the most common type of neurocognitive disorder. Although both amyloid β peptide deposition and neurofibrillary tangle formation in the AD brain have been established as pathological hallmarks of the disease, many other factors contribute in a complex manner to the pathogenesis of AD before clinical symptoms of the disease become apparent. Longitudinal pathophysiological processes cause patients' brains to exist in a state of chronic neuroinflammation, with glial cells acting as key regulators of the neuroinflammatory state. However, the detailed molecular and cellular mechanisms of glial function underlying AD pathogenesis remain elusive. Furthermore, recent studies have shown that peripheral inflammatory conditions affect glial cells in the brain through a process of neuroimmune communication. Such disease complexities make it difficult for the pathogenesis of AD to be understood, and impede the development of effective therapeutic strategies to combat the disease. Relevant AD animal models are thus likely to serve as a key resource to overcome many of these issues. Furthermore, as the pathogenesis of AD might be linked to conditions both within the brain as well as peripherally, it might become necessary for AD to be studied as a whole-body disorder. The present review aimed to summarize insights regarding current AD research, and share perspectives for understanding glial function in the context of the pathogenesis of AD.

Cognitive and emotional alterations in App knock-in mouse models of Aβ amyloidosis

Background

Alzheimer’s disease (AD), the most common cause of dementia, is characterized by the progressive deposition of amyloid-β (Aβ) peptides and neurofibrillary tangles. Mouse models of Aβ amyloidosis generated by knock-in (KI) of a humanized Aβ sequence provide distinct advantages over traditional transgenic models that rely on overexpression of amyloid precursor protein (APP). In App-KI mice, three familial AD-associated mutations were introduced into the endogenous mouse App locus to recapitulate Aβ pathology observed in AD: the Swedish (NL) mutation, which elevates total Aβ production; the Beyreuther/Iberian (F) mutation, which increases the Aβ42/Aβ40 ratio; and the Arctic (G) mutation, which promotes Aβ aggregation. App^NL-G-F mice harbor all three mutations and develop progressive Aβ amyloidosis and neuroinflammatory response in broader brain areas, whereas App^NL mice carrying only the Swedish mutation exhibit no overt AD-related pathological changes. To identify behavioral alterations associated with Aβ pathology, we assessed emotional and cognitive domains of App^NL-G-F and App^NL mice at different time points, using the elevated plus maze, contextual fear conditioning, and Barnes maze tasks.

Results

Assessments of emotional domains revealed that, in comparison with wild-type (WT) C57BL/6J mice, App^{NL-G-F/NL-G-F} mice exhibited anxiolytic-like behavior that was detectable from 6 months of age. By contrast, App^NL/NL mice exhibited anxiogenic-like behavior from 15 months of age. In the contextual fear conditioning task, both App^NL/NL and App^{NL-G-F/NL-G-F} mice exhibited intact learning and memory up to 15–18 months of age, whereas App^{NL-G-F/NL-G-F} mice exhibited hyper-reactivity to painful stimuli. In the Barnes maze task, App^{NL-G-F/NL-G-F} mice exhibited a subtle decline in spatial learning ability at 8 months of age, but retained normal memory functions.

Conclusion

App^NL/NL and App^{NL-G-F/NL-G-F} mice exhibit behavioral changes associated with non-cognitive, emotional domains before the onset of definitive cognitive deficits. Our observations consistently indicate that App^{NL-G-F/NL-G-F} mice represent a model for preclinical AD. These mice are useful for the study of AD prevention rather than treatment after neurodegeneration.

Electronic supplementary material

The online version of this article (10.1186/s12868-018-0446-8) contains supplementary material, which is available to authorized users.