Membrane-anchored Aβ accelerates amyloid formation and exacerbates amyloid-associated toxicity in mice

Lipid imaging of Alzheimer's disease pathology

Alzheimer's disease (AD) affects one in eight individuals over 65 and poses an immense societal challenge. AD pathology is characterized by the formation of beta-amyloid plaques and Tau tangles in the brain. While some disease-modifying treatments targeting beta-amyloid are emerging, the exact chain of events underlying the pathogenesis of this disease remains unclear. Brain lipids have long been implicated in AD pathology, though their role in AD pathogenesis remains not fully resolved. Significant advancements in mass spectrometry imaging (MSI) allow to detail spatial lipid regulations in biological tissues at the low um scale. In this issue, Huang et al. resolve spatial lipid patterns in human AD brain and genetic mouse models using desorption electrospray ionization (DESI)-based MSI integrated with other spatial techniques such as imaging mass cytometry of correlative protein signatures. Those spatial multiomics experiments identify plaque-associated lipid regulations that are dependent on progressing plaque pathology in both mouse models and the human brain. Of those lipid species, particularly pro-inflammatory lysophospholipids have been implicated in AD pathology through their interaction with both aggregating Aβ and microglial activation through lipid sensing surface receptors. Together, this study provides further insight into how brain lipid homeostasis is linked to progressing AD pathology, and thereby highlights the potential of MSI-based spatial lipidomics as an emerging spatial biology technology for biomedical research.

Aggregation, structure and water permeability of membrane-embedded helical Aβ oligomers

It is widely recognized that membranes can facilitate the aggregation of amyloid-β (Aβ) peptides, while Aβ can in turn cause membrane damage. Many studies focus on the peptide-membrane interactions of Aβ oligomers with β-rich structures. However, the exact aggregation and toxicity mechanism of the membrane-embedded helical Aβ oligomers remain ambiguous. Herein, the molecular dynamics simulations were performed on membrane-embedded helical Aβ42 peptides. Initiated by eight Aβ42 monomers embedded in a lipid bilayer, the monomers aggregate into oligomers with stable transmembrane helix structures. With the aggregation of peptides, the membrane perturbations caused by Aβ aggregates decrease. The molecular architectures of oligomers were characterized and a helix-rich octamer stabilized by an annular network of hydrogen bonds was observed. The oligomers demonstrate the capability to assist transmembrane water transport. Our study may provide new insights for the investigation of transmembrane Aβ oligomers.

Evidence that Alzheimer's Disease Is a Disease of Competitive Synaptic Plasticity Gone Awry

 Mounting evidence indicates that a physiological function of amyloid-β (Aβ) is to mediate neural activity-dependent homeostatic and competitive synaptic plasticity in the brain. I have previously summarized the lines of evidence supporting this hypothesis and highlighted the similarities between Aβ and anti-microbial peptides in mediating cell/synapse competition. In cell competition, anti-microbial peptides deploy a multitude of mechanisms to ensure both self-protection and competitor elimination. Here I review recent studies showing that similar mechanisms are at play in Aβ-mediated synapse competition and perturbations in these mechanisms underpin Alzheimer's disease (AD). Specifically, I discuss evidence that Aβ and ApoE, two crucial players in AD, co-operate in the regulation of synapse competition. Glial ApoE promotes self-protection by increasing the production of trophic monomeric Aβ and inhibiting its assembly into toxic oligomers. Conversely, Aβ oligomers, once assembled, promote the elimination of competitor synapses via direct toxic activity and amplification of "eat-me" signals promoting the elimination of weak synapses. I further summarize evidence that neuronal ApoE may be part of a gene regulatory network that normally promotes competitive plasticity, explaining the selective vulnerability of ApoE expressing neurons in AD brains. Lastly, I discuss evidence that sleep may be key to Aβ-orchestrated plasticity, in which sleep is not only induced by Aβ but is also required for Aβ-mediated plasticity, underlining the link between sleep and AD. Together, these results strongly argue that AD is a disease of competitive synaptic plasticity gone awry, a novel perspective that may promote AD research.

LILRB2-mediated TREM2 signaling inhibition suppresses microglia functions

Background

Microglia plays crucial roles in Alzheimer’s disease (AD) development. Triggering receptor expressed on myeloid cells 2 (TREM2) in association with DAP12 mediates signaling affecting microglia function. Here we study the negative regulation of TREM2 functions by leukocyte immunoglobulin-like receptor subfamily B member 2 (LILRB2), an inhibitory receptor bearing ITIM motifs.

Methods

To specifically interrogate LILRB2-ligand (oAβ and PS) interactions and microglia functions, we generated potent antagonistic LILRB2 antibodies with sub-nanomolar level activities. The biological effects of LILRB2 antagonist antibody (Ab29) were studied in human induced pluripotent stem cell (iPSC)–derived microglia (hMGLs) for migration, oAβ phagocytosis, and upregulation of inflammatory cytokines. Effects of the LILRB2 antagonist antibody on microglial responses to amyloid plaques were further studied in vivo using stereotaxic grafted microglia in 5XFAD mice.

Results

We confirmed the expression of both LILRB2 and TREM2 in human brain microglia using immunofluorescence. Upon co-ligation of the LILRB2 and TREM2 by shared ligands oAβ or PS, TREM2 signaling was significantly inhibited. We identified a monoclonal antibody (Ab29) that blocks LILRB2/ligand interactions and prevents TREM2 signaling inhibition mediated by LILRB2. Further, Ab29 enhanced microglia phagocytosis, TREM2 signaling, migration, and cytokine responses to the oAβ-lipoprotein complex in hMGL and microglia cell line HMC3. In vivo studies showed significantly enhanced clustering of microglia around plaques with a prominent increase in microglial amyloid plaque phagocytosis when 5XFAD mice were treated with Ab29.

Conclusions

This study revealed for the first time the molecular mechanisms of LILRB2-mediated inhibition of TREM2 signaling in microglia and demonstrated a novel approach of enhancing TREM2-mediated microglia functions by blocking LILRB2-ligand interactions. Translationally, a LILRB2 antagonist antibody completely rescued the inhibition of TREM2 signaling by LILRB2, suggesting a novel therapeutic strategy for improving microglial functions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-022-00550-y.

The Association of Lipids with Amyloid Fibrils

Amyloid formation continues to be a widely studied area because of its association with numerous diseases, such as Alzheimer’s and Parkinson’s diseases. Despite a large body of work on protein aggregation and fibril formation, there are still significant gaps in our understanding of the factors that differentiate toxic amyloid formation in vivo from alternative misfolding pathways. In addition to proteins, amyloid fibrils are often associated in their cellular context with several types of molecule, including carbohydrates, polyanions, and lipids. This review focuses in particular on evidence for the presence of lipids in amyloid fibrils and the routes by which those lipids may become incorporated. Chemical analyses of fibril composition, combined with studies to probe the lipid distribution around fibrils, provide evidence that in some cases, lipids have a strong association with fibrils. In addition, amyloid fibrils formed in the presence of lipids have distinct morphologies and material properties. It is argued that lipids are an integral part of many amyloid deposits in vivo, where their presence has the potential to influence the nucleation, morphology, and mechanical properties of fibrils. The role of lipids in these structures is therefore worthy of further study.

Genetic Inactivation of Free Fatty Acid Receptor 3 Impedes Behavioral Deficits and Pathological Hallmarks in the APPswe Alzheimer's Disease Mouse Model

The free fatty acid FFA3 receptor (FFA3R) belongs to the superfamily of G-protein-coupled receptors (GPCRs). In the intestine and adipose tissue, it is involved in the regulation of energy metabolism, but its function in the brain is unknown. We aimed, first, to investigate the expression of the receptor in the hippocampus of Alzheimer disease (AD) patients at different stages of the disease and, second, to assess whether genetic inactivation of the Ffar3 gene could affect the phenotypic features of the APP_swe mouse model. The expression of transcripts for FFA receptors in postmortem human hippocampal samples and in the hippocampus of wild-type and transgenic mice was analyzed by RT-qPCR. We generated a double transgenic mouse, FFA3R^−/−/APP_swe, to perform cognition studies and to assess, by immunoblotting Aβ and tau pathologies and the differential expression of synaptic plasticity-related proteins. For the first time, the occurrence of the FFA3R in the human hippocampus and its overexpression, even in the first stages of AD, was demonstrated. Remarkably, FFA3R^−/−/APP_swe mice do not have the characteristic memory impairment of 12-month-old APP_swe mice. Additionally, this newly generated transgenic line does not develop the most important Alzheimer’s disease (AD)-related features, such as amyloid beta (Aβ) brain accumulations and tau hyperphosphorylation. These findings are accompanied by increased levels of the insulin-degrading enzyme (IDE) and lower activity of the tau kinases GSK3β and Cdk5. We conclude that the brain FFA3R is involved in cognitive processes and that its inactivation prevents AD-like cognitive decline and pathological hallmarks.

Transferrin-Pep63-liposomes accelerate the clearance of Aβ and rescue impaired synaptic plasticity in early Alzheimer's disease models

Alzheimer’s disease (AD) is characterized by aberrant accumulation of extracellular β-amyloid (Aβ) peptides in the brain. Soluble Aβ oligomers are thought to be the most neurotoxic species and are correlated with cognitive dysfunction in early AD. However, there is still no effective treatment so far. We determined that Pep63, a small peptide, had a neuroprotective effect on synaptic plasticity and memory in our previous study. Here, we developed novel and multifunctional liposomes targeting both Aβ oligomers and fibrils based on a liposome delivery system. Transferrin-Pep63-liposomes (Tf-Pep63-Lip), possessing the ability for blood-brain barrier targeting, were also incorporated with phosphatidic acid (PA) and loaded with neuroprotective Pep63. We discovered that administration of Tf-Pep63-Lip could significantly reduce the Aβ burden in the hippocampus, and improve cognitive deficits in 6-month-old APP/PS1 mice in the Morris-Water maze task and fear-conditioning test with the combined effects of PA and Pep63. Tf-Pep63-Lip could capture Aβ oligomers or fibrils and then facilitated microglial chemotaxis nearby for clearance. Simultaneously, Tf-Pep63-Lip hindered Aβ1-42 aggregation and disaggregated Aβ1-42 assembly due to multivalent PA-Aβ. Pep63 effectively inhibited the binding between EphB2 and Aβ oligomers after release from liposomes and rescued NMDA receptors trafficking, the basis of synaptic plasticity. No side effects were observed in either APP/PS1 or wild-type mice, indicating that Tf-Pep63-Lip might be safe under the dosing regimen used in our experiment. Taken together, our results suggested that Tf-Pep63-Lip may serve as a safe and efficient agent for AD combination therapy.

Neuroinflammatory Signaling in the Pathogenesis of Alzheimer's Disease

Alzheimer’s disease (AD) is a chronic neurodegenerative disease characterized by the formation of intracellular neurofibrillary tangles (NFTs) and extracellular amyloid plaques. Growing evidence has suggested that AD pathogenesis is not only limited to the neuronal compartment but also strongly interacts with immunological processes in the brain. On the other hand, aggregated and misfolded proteins can bind with pattern recognition receptors located on astroglia and microglia and can, in turn, induce an innate immune response, characterized by the release of inflammatory mediators, ultimately playing a role in both the severity and the progression of the disease. It has been reported by genome-wide analysis that several genes which elevate the risk for sporadic AD encode for factors controlling the inflammatory response and glial clearance of misfolded proteins. Obesity and systemic inflammation are examples of external factors which may interfere with the immunological mechanisms of the brain and can induce disease progression. In this review, we discussed the mechanisms and essential role of inflammatory signaling pathways in AD pathogenesis. Indeed, interfering with immune processes and modulation of risk factors may lead to future therapeutic or preventive AD approaches.

Ultrasmall Molybdenum Disulfide Quantum Dots Cage Alzheimer's Amyloid Beta to Restore Membrane Fluidity

Alzheimer's disease (AD) is a major cause of dementia characterized by the overexpression of transmembrane amyloid precursor protein and its neurotoxic byproduct amyloid beta (Aβ). A small peptide of considerable hydrophobicity, Aβ is aggregation prone catalyzed by the presence of cell membranes, among other environmental factors. Accordingly, current AD mitigation strategies often aim at breaking down the Aβ-membrane communication, yet no data is available concerning the cohesive interplay of the three key entities of the cell membrane, Aβ, and its inhibitor. Using a lipophilic Laurdan dye and confocal fluorescence microscopy, we observed cell membrane perturbation and actin reorganization induced by Aβ oligomers but not by Aβ monomers or amyloid fibrils. We further revealed recovery of membrane fluidity by ultrasmall MoS2 quantum dots, also shown in this study as a potent inhibitor of Aβ amyloid aggregation. Using discrete molecular dynamics simulations, we uncovered the binding of MoS2 and Aβ monomers as mediated by hydrophilic interactions between the quantum dots and the peptide N-terminus. In contrast, Aβ oligomers and fibrils were surface-coated by the ultrasmall quantum dots in distinct testudo-like, reverse protein-corona formations to prevent their further association with the cell membrane and adverse effects downstream. This study offers a crucial new insight and a viable strategy for regulating the amyloid aggregation and membrane-axis of AD pathology with multifunctional nanomedicine.

Application of DNP-enhanced solid-state NMR to studies of amyloid-β peptide interaction with lipid membranes

The cellular membrane disruption induced by the aggregation of Aβ peptide has been proposed as a plausible cause of neuronal cell death during Alzheimer's disease. The molecular-level details of the Aβ interaction with cellular membranes were previously probed using solid state NMR (ssNMR), however, due to the limited sensitivity of the latter, studies were limited to samples with high Aβ-to-lipid ratio. The dynamic nuclear polarization (DNP) is a technique for increasing the sensitivity of NMR. In this work we demonstrate the feasibility of DNP-enhanced ssNMR studies of Aβ₄₀ peptide interacting with various model liposomes: (1) a mixture of zwitterionic 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) and negatively charged 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG); (2) a mixture of POPC, POPG, cholesterol, sphingomyelin and ganglioside GM1; (3) the synaptic plasma membrane vesicles (SPMVs) extracted from rat brain tissues. In addition, DNP-ssNMR was applied to capturing changes in Aβ₄₀ conformation taking place upon the peptide insertion into POPG liposomes. The signal enhancements under conditions of DNP allow carrying out informative 2D ssNMR experiments with about 0.25 mg of Aβ₄₀ peptides (i.e. reaching Aβ₄₀-to-lipid ratio of 1:200). In the studied liposome models, the ¹³C NMR chemical shifts at many ¹³C-labelled sites of Aβ₄₀ are characteristic of β-sheets. In addition, in POPG liposomes the peptide forms hydrophobic contacts F19-L34 and F19-I32. Both the chemical shifts and hydrophobic contacts of Aβ₄₀ in POPG remain the same before and after 8 h of incubation. This suggests that conformation at the ¹³C-labelled sites of the peptide is similar before and after the insertion process. Overall, our results demonstrate that DNP helps to overcome the sensitivity limitation of ssNMR, and thereby expand the applicability of ssNMR for charactering the Aβ peptide interacting with lipids.

Length-Dependent Manifestation of Vibration Modes Regulates a Specific Intermediate Morphology of Aβ17-42 in Different Environments

Various cytotoxic mechanisms for neurodegenerative disease are induced by specific conformations of Aβ intermediates. The efforts to understand the diverse intermediate forms of amyloid oligomers have been focused on understanding the aggregation mechanism of specific morphologies for Aβ intermediates. However, these are still not easy tasks to be accomplished because the diverse conformations of Aβ intermediates can be altered during the aggregation process, even though the same Aβ monomers are present. Thus, efforts to reveal the conformational change mechanism could be a fundamental process to understand the formation of diverse Aβ intermediate conformations. Here, we evaluate the conformational characteristics of Aβ_17-42 fibrillar oligomers in different environments according to the length. We observed that Aβ fibrillar oligomers optimize their inherent hydrogen bonds and configurational entropy to stabilize their structure according to the simulation time and their length increase. In addition, we revealed the role of the expressed vibration mode shape in the fibrillar oligomers' elongation and deformation processes. Our results suggest that limitations in amyloid oligomer growth and transformations of their morphologies can be regulated and controlled by modifying the vibration features.

Prion-like propagation of β-amyloid aggregates in the absence of APP overexpression

The amyloid cascade hypothesis posits that the initiating event in Alzheimer's disease (AD) is the aggregation and deposition of the β-amyloid (Aβ) peptide, which is a proteolytic cleavage product of the amyloid precursor protein (APP). Mounting evidence suggests that the formation and spread of prion-like Aβ aggregates during AD may contribute to disease progression. Inoculation of transgenic mice that overexpress APP with pre-formed Aβ aggregates results in the prion-like induction of cerebral Aβ deposition. To determine whether Aβ deposition can also be induced when physiological APP levels are present in the brain, we inoculated App^NL-F mice, a knock-in model of AD that avoids potential artifacts associated with APP overexpression, with Aβ aggregates derived from the brains of AD patients or transgenic mice. In all cases, induced Aβ deposition was apparent in the corpus callosum, olfactory bulb, and meningeal blood vessels of inoculated mice at 130-150 days post-inoculation, whereas uninoculated and buffer-inoculated animals exhibited minimal or no Aβ deposits at these ages. Interestingly, despite being predominantly composed of protease-resistant Aβ42 aggregates, the induced parenchymal Aβ deposits were largely diffuse and were unreactive to an amyloid-binding dye. These results demonstrate that APP overexpression is not a prerequisite for the prion-like induction of cerebral Aβ deposition. Accordingly, spreading of Aβ deposition may contribute to disease progression in AD patients.

Cell model for the identification and characterization of prion-like components from Alzheimer brain tissue

Intracerebral injection of brain extracts from Alzheimer's disease (AD) patients into appropriate mouse models was previously found to drastically accelerate the deposition of Aβ amyloid in the recipient animals indicating a prion-like activity. In this study we show that this prion-like activity can be also identified by using a cell culture model of Aβ plaque formation. Analysis of biochemical fractions of AD brain extract indicate that the seeding-activity correlated with the presence of Aβ peptide and Aβ-derived aggregates. In vitro-formed fibrils were also active but their activity was low and depending on the fibril structure and conditions of fibril formation. Our data indicate a conformational basis of the observed seeding effect and suggest the utility of our cell model for further studies on the prion-like activity of AD extracts.

Changes in the detergent-insoluble brain proteome linked to amyloid and tau in Alzheimer's Disease progression

Despite a key role of amyloid-beta (Aβ) in Alzheimer's disease (AD), mechanisms that link Aβ plaques to tau neurofibrillary tangles and cognitive decline still remain poorly understood. The purpose of this study was to quantify proteins in the sarkosyl-insoluble brain proteome correlated with Aβ and tau insolubility in the asymptomatic phase of AD (AsymAD) and through mild cognitive impairment (MCI) and symptomatic AD. Employing label-free mass spectrometry-based proteomics, we quantified 2711 sarkosyl-insoluble proteins across the prefrontal cortex from 35 individual cases representing control, AsymAD, MCI and AD. Significant enrichment of Aβ and tau in AD was observed, which correlated with neuropathological measurements of plaque and tau tangle density, respectively. Pairwise correlation coefficients were also determined for all quantified proteins to Aβ and tau, across the 35 cases. Notably, six of the ten most correlated proteins to Aβ were U1 small nuclear ribonucleoproteins (U1 snRNPs). Three of these U1 snRNPs (U1A, SmD and U1-70K) also correlated with tau consistent with their association with tangle pathology in AD. Thus, proteins that cross-correlate with both Aβ and tau, including specific U1 snRNPs, may have potential mechanistic roles in linking Aβ plaques to tau tangle pathology during AD progression.

Age-Dependent Protein Aggregation Initiates Amyloid-β Aggregation

Aging is the most important risk factor for neurodegenerative diseases associated with pathological protein aggregation such as Alzheimer's disease. Although aging is an important player, it remains unknown which molecular changes are relevant for disease initiation. Recently, it has become apparent that widespread protein aggregation is a common feature of aging. Indeed, several studies demonstrate that 100s of proteins become highly insoluble with age, in the absence of obvious disease processes. Yet it remains unclear how these misfolded proteins aggregating with age affect neurodegenerative diseases. Importantly, several of these aggregation-prone proteins are found as minor components in disease-associated hallmark aggregates such as amyloid-β plaques or neurofibrillary tangles. This co-localization raises the possibility that age-dependent protein aggregation directly contributes to pathological aggregation. Here, we show for the first time that highly insoluble proteins from aged Caenorhabditis elegans or aged mouse brains, but not from young individuals, can initiate amyloid-β aggregation in vitro. We tested the seeding potential at four different ages across the adult lifespan of C. elegans. Significantly, protein aggregates formed during the early stages of aging did not act as seeds for amyloid-β aggregation. Instead, we found that changes in protein aggregation occurring during middle-age initiated amyloid-β aggregation. Mass spectrometry analysis revealed several late-aggregating proteins that were previously identified as minor components of amyloid-β plaques and neurofibrillary tangles such as 14-3-3, Ubiquitin-like modifier-activating enzyme 1 and Lamin A/C, highlighting these as strong candidates for cross-seeding. Overall, we demonstrate that widespread protein misfolding and aggregation with age could be critical for the initiation of pathogenesis, and thus should be targeted by therapeutic strategies to alleviate neurodegenerative diseases.

Highly potent intracellular membrane-associated Aβ seeds

An early event in Alzheimer's disease (AD) pathogenesis is the formation of extracellular aggregates of amyloid-β peptide (Aβ), thought to be initiated by a prion-like seeding mechanism. However, the molecular nature and location of the Aβ seeds remain rather elusive. Active Aβ seeds are found in crude homogenates of amyloid-laden brains and in the soluble fraction thereof. To analyze the seeding activity of the pellet fraction, we have either separated or directly immunoisolated membranes from such homogenates. Here, we found considerable Aβ seeding activity associated with membranes in the absence of detectable amyloid fibrils. We also found that Aβ seeds on mitochondrial or associated membranes efficiently induced Aβ aggregation in vitro and seed β-amyloidosis in vivo. Aβ seeds at intracellular membranes may contribute to the spreading of Aβ aggregation along neuronal pathways and to the induction of intracellular pathologies downstream of Aβ.

Platelets contribute to amyloid-β aggregation in cerebral vessels through integrin αIIbβ3-induced outside-in signaling and clusterin release

Cerebral amyloid angiopathy (CAA) is a vascular dysfunction disorder characterized by deposits of amyloid-β (Aβ) in the walls of cerebral vessels. CAA and Aβ deposition in the brain parenchyma contribute to dementia and Alzheimer's disease (AD). We investigated the contribution of platelets, which accumulate at vascular Aβ deposits, to CAA. We found that synthetic monomeric Aβ40 bound through its RHDS (Arg-His-Asp-Ser) sequence to integrin αIIbβ3, which is the receptor for the extracellular matrix protein fibrinogen, and stimulated the secretion of adenosine diphosphate (ADP) and the chaperone protein clusterin from platelets. Clusterin promoted the formation of fibrillar Aβ aggregates, and ADP acted through its receptors P2Y1 and P2Y12 on platelets to enhance integrin αIIbβ3 activation, further increasing the secretion of clusterin and Aβ40 binding to platelets. Platelets from patients with Glanzmann's thrombasthenia, a bleeding disorder in which platelets have little or dysfunctional αIIbβ3, indicated that the abundance of this integrin dictated Aβ-induced clusterin release and platelet-induced Aβ aggregation. The antiplatelet agent clopidogrel, which irreversibly inhibits P2Y12, inhibited Aβ aggregation in platelet cultures; in transgenic AD model mice, this drug reduced the amount of clusterin in the circulation and the incidence of CAA. Our findings indicate that activated platelets directly contribute to CAA by promoting the formation of Aβ aggregates and that Aβ, in turn, activates platelets, creating a feed-forward loop. Thus, antiplatelet therapy may alleviate fibril formation in cerebral vessels of AD patients.

Conversion of Synthetic Aβ to In Vivo Active Seeds and Amyloid Plaque Formation in a Hippocampal Slice Culture Model

The aggregation of amyloid-β peptide (Aβ) in brain is an early event and hallmark of Alzheimer's disease (AD). We combined the advantages of in vitro and in vivo approaches to study cerebral β-amyloidosis by establishing a long-term hippocampal slice culture (HSC) model. While no Aβ deposition was noted in untreated HSCs of postnatal Aβ precursor protein transgenic (APP tg) mice, Aβ deposition emerged in HSCs when cultures were treated once with brain extract from aged APP tg mice and the culture medium was continuously supplemented with synthetic Aβ. Seeded Aβ deposition was also observed under the same conditions in HSCs derived from wild-type or App-null mice but in no comparable way when HSCs were fixed before cultivation. Both the nature of the brain extract and the synthetic Aβ species determined the conformational characteristics of HSC Aβ deposition. HSC Aβ deposits induced a microglia response, spine loss, and neuritic dystrophy but no obvious neuron loss. Remarkably, in contrast to in vitro aggregated synthetic Aβ, homogenates of Aβ deposits containing HSCs induced cerebral β-amyloidosis upon intracerebral inoculation into young APP tg mice. Our results demonstrate that a living cellular environment promotes the seeded conversion of synthetic Aβ into a potent in vivo seeding-active form.

SIGNIFICANCE STATEMENT

In this study, we report the seeded induction of Aβ aggregation and deposition in long-term hippocampal slice cultures. Remarkably, we find that the biological activities of the largely synthetic Aβ aggregates in the culture are very similar to those observed in vivo This observation is the first to show that potent in vivo seeding-active Aβ aggregates can be obtained by seeded conversion of synthetic Aβ in a living (wild-type) cellular environment.

Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23

Spinocerebellar ataxia type 23 is caused by mutations in PDYN, which encodes the opioid neuropeptide precursor protein, prodynorphin. Prodynorphin is processed into the opioid peptides, α-neoendorphin, and dynorphins A and B, that normally exhibit opioid-receptor mediated actions in pain signalling and addiction. Dynorphin A is likely a mutational hotspot for spinocerebellar ataxia type 23 mutations, and in vitro data suggested that dynorphin A mutations lead to persistently elevated mutant peptide levels that are cytotoxic and may thus play a crucial role in the pathogenesis of spinocerebellar ataxia type 23. To further test this and study spinocerebellar ataxia type 23 in more detail, we generated a mouse carrying the spinocerebellar ataxia type 23 mutation R212W in PDYN. Analysis of peptide levels using a radioimmunoassay shows that these PDYN(R212W) mice display markedly elevated levels of mutant dynorphin A, which are associated with climber fibre retraction and Purkinje cell loss, visualized with immunohistochemical stainings. The PDYN(R212W) mice reproduced many of the clinical features of spinocerebellar ataxia type 23, with gait deficits starting at 3 months of age revealed by footprint pattern analysis, and progressive loss of motor coordination and balance at the age of 12 months demonstrated by declining performances on the accelerating Rotarod. The pathologically elevated mutant dynorphin A levels in the cerebellum coincided with transcriptionally dysregulated ionotropic and metabotropic glutamate receptors and glutamate transporters, and altered neuronal excitability. In conclusion, the PDYN(R212W) mouse is the first animal model of spinocerebellar ataxia type 23 and our work indicates that the elevated mutant dynorphin A peptide levels are likely responsible for the initiation and progression of the disease, affecting glutamatergic signalling, neuronal excitability, and motor performance. Our novel mouse model defines a critical role for opioid neuropeptides in spinocerebellar ataxia, and suggests that restoring the elevated mutant neuropeptide levels can be explored as a therapeutic intervention.

TREM2 lipid sensing sustains the microglial response in an Alzheimer's disease model

Triggering receptor expressed on myeloid cells 2 (TREM2) is a microglia surface receptor that triggers intracellular protein tyrosine phosphorylation. Recent genome-wide association studies have shown that a rare R47H mutation of TREM2 correlates with a substantial increase in the risk of developing Alzheimer's disease (AD). To address the basis for this genetic association, we studied TREM2 deficiency in the 5XFAD mouse model of AD. We found that TREM2 deficiency and haploinsufficiency augment β-amyloid (Aβ) accumulation due to dysfunctional response of microglia, which become apoptotic and fail to cluster around Aβ plaques. We further demonstrate that TREM2 senses a broad array of anionic and zwitterionic lipids known to associate with fibrillar Aβ in lipid membranes and to be exposed on the surface of damaged neurons. Remarkably, the R47H mutation impairs TREM2 detection of lipid ligands. Thus, TREM2 detects damage-associated lipid patterns associated with neurodegeneration, sustaining microglia response to Aβ accumulation.

Aβ seeds resist inactivation by formaldehyde

Cerebral β-amyloidosis can be exogenously induced by the intracerebral injection of brain extracts containing aggregated β-amyloid (Aβ) into young, pre-depositing Aβ precursor protein- (APP) transgenic mice. Previous work has shown that the induction involves a prion-like seeding mechanism in which the seeding agent is aggregated Aβ itself. Here we report that the β-amyloid-inducing activity of Alzheimer's disease (AD) brain tissue or aged APP-transgenic mouse brain tissue is preserved, albeit with reduced efficacy, after formaldehyde fixation. Moreover, spectral analysis with amyloid conformation-sensitive luminescent conjugated oligothiophene dyes reveals that the strain-like properties of aggregated Aβ are maintained in fixed tissues. The resistance of Aβ seeds to inactivation and structural modification by formaldehyde underscores their remarkable durability, which in turn may contribute to their persistence and spread within the body. The present findings can be exploited to establish the relationship between the molecular structure of Aβ aggregates and the variable clinical features and disease progression of AD even in archived, formalin-fixed autopsy material.

Membrane pathology and microglial activation of mice expressing membrane anchored or membrane released forms of Aβ and mutated human Alzheimer's precursor protein (APP)

AIMS

Alzheimer's disease and the transmissible spongiform encephalopathies or prion diseases accumulate misfolded and aggregated forms of neuronal cell membrane proteins. Distinctive membrane lesions caused by the accumulation of disease-associated prion protein (PrP(d)) are found in prion disease but morphological changes of membranes are not associated with Aβ in Alzheimer's disease. Membrane changes occur in all prion diseases where PrP(d) is attached to cell membranes by a glycosyl-phosphoinositol (GPI) anchor but are absent from transgenic mice expressing anchorless PrP(d). Here we investigate whether GPI membrane attached Aβ may also cause prion-like membrane lesions.

METHODS

We used immunogold electron microscopy to determine the localization and pathology of Aβ accumulation in groups of transgenic mice expressing anchored or unanchored forms of Aβ or mutated human Alzheimer's precursor protein.

RESULTS

GPI attached Aβ did not replicate the membrane lesions of PrP(d). However, as with PrP(d) in prion disease, Aβ peptides derived from each transgenic mouse line initially accumulated on morphologically normal neurite membranes, elicited rapid glial recognition and neurite Aβ was transferred to attenuated microglial and astrocytic processes.

CONCLUSIONS

GPI attachment of misfolded membrane proteins is insufficient to cause prion-like membrane lesions. Prion disease and murine Aβ amyloidosis both accumulate misfolded monomeric or oligomeric membrane proteins that are recognized by glial processes and acquire such misfolded proteins prior to their accumulation in the extracellular space. In contrast to prion disease where glial cells efficiently endocytose PrP(d) to endolysosomes, activated microglial cells in murine Aβ amyloidosis are not as efficient phagocytes.