Diffusible, highly bioactive oligomers represent a critical minority of soluble Aβ in Alzheimer's disease brain

OUP accepted manuscript

Aqueously soluble oligomers of amyloid-β peptide may be the principal neurotoxic forms of amyloid-β in Alzheimer’s disease, initiating downstream events that include tau hyperphosphorylation, neuritic/synaptic injury, microgliosis and neuron loss. Synthetic oligomeric amyloid-β has been studied extensively, but little is known about the biochemistry of natural oligomeric amyloid-β in human brain, even though it is more potent than simple synthetic peptides and comprises truncated and modified amyloid-β monomers. We hypothesized that monoclonal antibodies specific to neurotoxic oligomeric amyloid-β could be used to isolate it for further study.

Here we report a unique human monoclonal antibody (B24) raised against synthetic oligomeric amyloid-β that potently prevents Alzheimer’s disease brain oligomeric amyloid-β-induced impairment of hippocampal long-term potentiation. B24 binds natural and synthetic oligomeric amyloid-β and a subset of amyloid plaques, but only in the presence of Ca²⁺. The amyloid-β N terminus is required for B24 binding. Hydroxyapatite chromatography revealed that natural oligomeric amyloid-β is highly avid for Ca²⁺. We took advantage of the reversible Ca²⁺-dependence of B24 binding to perform non-denaturing immunoaffinity isolation of oligomeric amyloid-β from Alzheimer’s disease brain-soluble extracts.

Unexpectedly, the immunopurified material contained amyloid fibrils visualized by electron microscopy and amenable to further structural characterization. B24-purified human oligomeric amyloid-β inhibited mouse hippocampal long-term potentiation. These findings identify a calcium-dependent method for purifying bioactive brain oligomeric amyloid-β, at least some of which appears fibrillar.

Reducing voltage-dependent potassium channel Kv3.4 levels ameliorates synapse loss in a mouse model of Alzheimer's disease

Synapse loss is associated with cognitive decline in Alzheimer's disease, and owing to their plastic nature, synapses are an ideal target for therapeutic intervention. Oligomeric amyloid beta around amyloid plaques is known to contribute to synapse loss in mouse models and is associated with synapse loss in human Alzheimer's disease brain tissue, but the mechanisms leading from Aβ to synapse loss remain unclear. Recent data suggest that the fast-activating and -inactivating voltage-gated potassium channel subtype 3.4 (Kv3.4) may play a role in Aβ-mediated neurotoxicity. Here, we tested whether this channel could also be involved in Aβ synaptotoxicity. Using adeno-associated virus and clustered regularly interspaced short palindromic repeats technology, we reduced Kv3.4 expression in neurons of the somatosensory cortex of APP/PS1 mice. These mice express human familial Alzheimer's disease-associated mutations in amyloid precursor protein and presenilin-1 and develop amyloid plaques and plaque-associated synapse loss similar to that observed in Alzheimer's disease brain. We observe that reducing Kv3.4 levels ameliorates dendritic spine loss and changes spine morphology compared to control virus. In support of translational relevance, Kv3.4 protein was observed in human Alzheimer's disease and control brain and is associated with synapses in human induced pluripotent stem cell-derived cortical neurons. We also noted morphological changes in induced pluripotent stem cell neurones challenged with human Alzheimer's disease-derived brain homogenate containing Aβ but, in this in vitro model, total mRNA levels of Kv3.4 were found to be reduced, perhaps as an early compensatory mechanism for Aβ-induced damage. Overall, our results suggest that approaches to reduce Kv3.4 expression and/or function in the Alzheimer's disease brain could be protective against Aβ-induced synaptic alterations.

Unbiased high-content screening reveals Aβ- and tau-independent synaptotoxic activities in human brain homogenates from Alzheimer's patients and high-pathology controls

Alzheimer’s disease (AD) is tightly correlated with synapse loss in vulnerable brain regions. It is assumed that specific molecular entities such as Aβ and tau cause synapse loss in AD, yet unbiased screens for synaptotoxic activities have not been performed. Here, we performed size exclusion chromatography on soluble human brain homogenates from AD cases, high pathology non-demented controls, and low pathology age-matched controls using our novel high content primary cultured neuron-based screening assay. Both presynaptic and postsynaptic toxicities were elevated in homogenates from AD cases and high pathology non-demented controls to a similar extent, with more modest synaptotoxic activities in homogenates from low pathology normal controls. Surprisingly, synaptotoxic activities were found in size fractions peaking between the 17–44 kDa size standards that did not match well with Aβ and tau immunoreactive species in these homogenates. The fractions containing previously identified high molecular weight soluble amyloid beta aggregates/”oligomers” were non-toxic in this assay. Furthermore, immunodepletion of Aβ and tau did not reduce synaptotoxic activity. This result contrasts with previous findings involving the same methods applied to 3xTg-AD mouse brain extracts. The nature of the synaptotoxic species has not been identified. Overall, our data indicates one or more potential Aβ and tau independent synaptotoxic activities in human AD brain homogenates. This result aligns well with the key role of synaptic loss in the early cognitive decline and may provide new insight into AD pathophysiology.

Tau-PET imaging as a molecular modality for Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative condition. The definitive diagnosis of AD remains a post-mortem neuropathological study of the brain. Unfortunately, there are no established diagnostic criteria to achieve an accurate diagnosis of AD in a similarly objective fashion among living patients. Molecular imaging provides one way of enhancing clinical criteria where objective measures of AD correlate to the presence and progression of disease. In this article, the amyloid and tau hypotheses are considered with respect to pathological, imaging, and therapeutic studies. The value of beta-amyloid (Aβ) PET and tau PET are ascertained. Subsequently, the binding characteristics and quality of Aβ and tau tracers are explored. Finally, the value of Aβ and tau imaging in AD can be determined relevant from in-vivo studies of AD patients. Considering the evolving literature in AD and PET imaging, it has become clear that PET can play a role in the diagnosis and prognosis of AD. The use of Aβ imaging has been extensively studied with mixed results suggesting a limited clinical utility. Conversely, tau-PET has shown early success in similar applications as Aβ imaging. Specifically, we find that there is value in FDG-PET and prospective utility in tau-PET. Ultimately, the community must acknowledge that the role of Aβ imaging for diagnosing and managing AD is very limited and that FDG-PET will remain the study of choice at this time. Moreover, research efforts must continue to determine the prospective value of tau imaging to the assessment of this disease.

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.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

DISCUSSION

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

Curcumin improves memory deficits by inhibiting HMGB1-RAGE/TLR4-NF-κB signalling pathway in APPswe/PS1dE9 transgenic mice hippocampus

Amyloid‐β (Aβ) deposition in the brain has been implicated in the development of Alzheimer's disease (AD), and neuroinflammation generates AD progression. Therapeutic effects of anti‐inflammatory approaches in AD are still under investigation. Curcumin, a potent anti‐inflammatory and antioxidant, has demonstrated therapeutic potential in AD models. However, curcumin's anti‐inflammatory molecular mechanisms and its associated cognitive impairment mechanisms in AD remain unclear. The high‐mobility group box‐1 protein (HMGB1) participates in the regulation of neuroinflammation. Herein, we attempted to evaluate the anti‐inflammatory effects of chronic oral administration of curcumin and HMGB1 expression in APP/PS1 transgenic mice AD model. We found that transgenic mice treated with a curcumin diet had shorter escape latencies and showed a significant increase in percent alternation, when compared with transgenic mice, in the Morris water maze and Y‐maze tests. Additionally, curcumin treatment could effectively decrease HMGB1 protein expression, advanced glycosylation end product‐specific receptor (RAGE), Toll‐like receptors‐4 (TLR4) and nuclear factor kappa B (NF‐κB) in transgenic mice hippocampus. However, amyloid plaques detected with thioflavin‐S staining in transgenic mice hippocampus were not affected by curcumin treatment. In contrast, curcumin significantly decreased GFAP‐positive cells, as assessed by immunofluorescence staining. Taken together, these data indicate that oral administration of curcumin may be a promising agent to attenuate memory deterioration in AD mice, probably inhibiting the HMGB1‐RAGE/TLR4‐NF‐κB inflammatory signalling pathway.

Soluble amyloid beta-containing aggregates are present throughout the brain at early stages of Alzheimer's disease

Protein aggregation likely plays a key role in the initiation and spreading of Alzheimer's disease pathology through the brain. Soluble aggregates of amyloid beta are believed to play a key role in this process. However, the aggregates present in humans are still poorly characterized due to a lack of suitable methods required for characterizing the low concentration of heterogeneous aggregates present. We have used a variety of biophysical methods to characterize the aggregates present in human Alzheimer's disease brains at Braak stage III. We find soluble amyloid beta-containing aggregates in all regions of the brain up to 200 nm in length, capable of causing an inflammatory response. Rather than aggregates spreading through the brain as disease progresses, it appears that aggregation occurs all over the brain and that different brain regions are at earlier or later stages of the same process, with the later stages causing increased inflammation.

Severe oligomeric tau toxicity can be reversed without long-term sequelae

Tau is a microtubule stabilizing protein that forms abnormal aggregates in many neurodegenerative disorders, including Alzheimer's disease. We have previously shown that co-expression of fragmented and full-length tau in P301SxTAU62on tau transgenic mice results in the formation of oligomeric tau species and causes severe paralysis. This paralysis is fully reversible once expression of the tau fragment is halted, even though P301S tau expression is maintained. Whereas various strategies to target tau aggregation have been developed, little is known about the long-term consequences of reverted tau toxicity. Therefore, we studied the long-term motor fitness of recovered, formerly paralysed P301SxTAU62on-off mice. To assess the seeding competence of oligomeric toxic tau species, we also inoculated ALZ17 mice with brainstem homogenates from paralysed P301SxTAU62on mice. Counter-intuitively, after recovery from paralysis due to oligomeric tau species expression, ageing P301SxTAU62on-off mice did not develop more motor impairment or tau pathology when compared to heterozygous P301S tau transgenic littermates. Thus, toxic tau species causing extensive neuronal dysfunction can be cleared without inducing seeding effects. Moreover, these toxic tau species also lack long-term tau seeding effects upon intrahippocampal inoculation into ALZ17 mice. In conclusion, tau species can be neurotoxic in the absence of seeding-competent tau aggregates, and mice can clear these tau forms permanently without tau seeding or spreading effects. These observations suggest that early targeting of non-fibrillar tau species may represent a therapeutically effective intervention in tauopathies. On the other hand, the absent seeding competence of early toxic tau species also warrants caution when using seeding-based tests for preclinical tauopathy diagnostics.

Laboratory evolution of a sortase enzyme that modifies amyloid-β protein

Epitope-specific enzymes are powerful tools for site-specific protein modification but generally require genetic manipulation of the target protein. Here, we describe the laboratory evolution of the bacterial transpeptidase sortase A to recognize the LMVGG sequence in endogenous amyloid-β (Aβ) protein. Using a yeast display selection for covalent bond formation, we evolved a sortase variant that prefers LMVGG substrates from a starting enzyme that prefers LPESG substrates, resulting in a >1,400-fold change in substrate preference. We used this evolved sortase to label endogenous Aβ in human cerebrospinal fluid, enabling the detection of Aβ with sensitivities rivaling those of commercial assays. The evolved sortase can conjugate a hydrophilic peptide to Aβ₄₂, greatly impeding the ability of the resulting protein to aggregate into higher-order structures. These results demonstrate laboratory evolution of epitope-specific enzymes toward endogenous targets as a strategy for site-specific protein modification without target gene manipulation and enable potential future applications of sortase-mediated labeling of Aβ peptides.

Quantifying misfolded protein oligomers as drug targets and biomarkers in Alzheimer and Parkinson diseases

Protein misfolding and aggregation are characteristic of a wide range of neurodegenerative disorders, including Alzheimer and Parkinson diseases. A hallmark of these diseases is the aggregation of otherwise soluble and functional proteins into amyloid aggregates. Although for many decades such amyloid deposits have been thought to be responsible for disease progression, it is now increasingly recognized that the misfolded protein oligomers formed during aggregation are, instead, the main agents causing pathological processes. These oligomers are transient and heterogeneous, which makes it difficult to detect and quantify them, generating confusion about their exact role in disease. The lack of suitable methods to address these challenges has hampered efforts to investigate the molecular mechanisms of oligomer toxicity and to develop oligomer-based diagnostic and therapeutic tools to combat protein misfolding diseases. In this Review, we describe methods to quantify misfolded protein oligomers, with particular emphasis on diagnostic applications as disease biomarkers and on therapeutic applications as target biomarkers. The development of these methods is ongoing, and we discuss the challenges that remain to be addressed to establish measurement tools capable of overcoming existing limitations and to meet present needs.

Degradation Products of Amyloid Protein: Are They The Culprits?

Objectives:

Beta-amyloid (Aβ) peptides are most toxic to cells in oligomeric form. It is commonly accepted that oligomers can form ion channels in cell membranes and allow calcium and other ions to enter cells. The activation of other mechanisms, such as apoptosis or lipid peroxidation, aggravates the toxicity, but it itself can result from the same initial point, that is, ion disturbance due to an increased permeability of membranes. However, experimental studies of membrane channels created by Aβ are surprisingly limited. Methods: Here, we report a novel flow cytometry technique which can be used to detect increased permeability of membranes to calcium induced by the exposure to amyloid peptides. Calcium entry into the liposome is monitored using calcium-sensitive fluorescent probe. Undamaged lipid membranes are not permeable to calcium. Liposomes that are prepared in a calcium-free medium become able to accumulate calcium in a calcium-containing medium only after the formation of channels. Using this technique, we demonstrated that the addition of short amyloid fragment Aβ, which is known for its extreme toxicity on cultured neurons, readily increased membrane permeability to calcium. However, neither similarly sized peptide Ab22-35 nor full-length peptide Ab1-42 were producing channels. The formation of channels was observed in the membranes made of phosphatidylserine, a negatively charged lipid, but not in membranes made of the neutral phosphatidylcholine.

Methods:

Here, we report a novel flow cytometry technique which can be used to detect increased permeability of membranes to calcium induced by the exposure to amyloid peptides. Calcium entry into the liposome is monitored using calcium-sensitive fluorescent probe. Undamaged lipid membranes are not permeable to calcium. Liposomes that are prepared in a calcium-free medium become able to accumulate calcium in a calcium-containing medium only after the formation of channels. Using this technique, we demonstrated that the addition of short amyloid fragment Aβ, which is known for its extreme toxicity on cultured neurons, readily increased membrane permeability to calcium. However, neither similarly sized peptide Ab22-35 nor full-length peptide Ab1-42 were producing channels. The formation of channels was observed in the membranes made of phosphatidylserine, a negatively charged lipid, but not in membranes made of the neutral phosphatidylcholine.

Results:

In the Discussion section, we have analyzed several issues which could be critical for understanding the pathogenesis of Alzheimer’s disease, specifically 1) the need for a negatively charged membrane to produce the ion channel; 2) the potential role of the aggregated form in cellular toxicity of Ab peptides; 3) channel-forming ability of multiple degradation products of amyloid; 4) non-specificity of ion channels formed by amyloid peptides. Potential targets of channel-forming oligomers appear to be intracellular and are organelles well-known for dysfunction in Alzheimer’s disease (mitochondria and lysosomes). In fact, lysosomes can also be the producers of degraded amyloid. Provided speculations support the hypothesis that neuronal toxicity can be caused by the degradation products of beta-amyloid.

Current and future applications of induced pluripotent stem cell-based models to study pathological proteins in neurodegenerative disorders

Neurodegenerative disorders emerge from the failure of intricate cellular mechanisms, which ultimately lead to the loss of vulnerable neuronal populations. Research conducted across several laboratories has now provided compelling evidence that pathogenic proteins can also contribute to non-cell autonomous toxicity in several neurodegenerative contexts, including Alzheimer's, Parkinson's, and Huntington's diseases as well as Amyotrophic Lateral Sclerosis. Given the nearly ubiquitous nature of abnormal protein accumulation in such disorders, elucidating the mechanisms and routes underlying these processes is essential to the development of effective treatments. To this end, physiologically relevant human in vitro models are critical to understand the processes surrounding uptake, release and nucleation under physiological or pathological conditions. This review explores the use of human-induced pluripotent stem cells (iPSCs) to study prion-like protein propagation in neurodegenerative diseases, discusses advantages and limitations of this model, and presents emerging technologies that, combined with the use of iPSC-based models, will provide powerful model systems to propel fundamental research forward.

APOE ε4/ε4 homozygotes with early Alzheimer's disease show accelerated hippocampal atrophy and cortical thinning that correlates with cognitive decline

INTRODUCTION

Hippocampal volume (HV) and cortical thickness are commonly used imaging biomarkers in Alzheimer's disease (AD) trials, and may have utility as selection criteria for enrichment strategies. Atrophy rates of these measures, in the high-risk apolipoprotein E (APOE) ε4/ε4 homozygous AD subjects are unknown.

METHODS

Data from Alzheimer's Disease Neuroimaging Initiative (ADNI-1) and a tramiprosate trial were analyzed in APOE ε4/ε4 and APOE ε3/ε3 subjects with mild cognitive impairment (MCI) or mild AD. Magnetic resonance imaging (MRI) data were centrally processed using FreeSurfer; total HV and composite average cortical thickness were derived and adjusted for age, head size, and education. Volumetric changes from baseline were assessed using Boundary Shift Integral, and correlated with cognitive changes.

RESULTS

APOE ε4/ε4 MCI subjects showed significantly higher % HV atrophy and cortical thinning at 12 months (4.4%, 3.1%, n = 29) compared to APOE ε3/ε3 subjects (2.8%, 1.8%, n = 93) and similarly in mild AD (7.4%, 4.7% n = 21 vs 5.4%, 3.3% n = 29). Differences were all significant at 24 months. Over 24 months, HV atrophy and cortical thinning correlated significantly with Alzheimer's Disease Assessment Scale-Cognitive subscale worsening in APOE ε4/ε4 MCI subjects, but not in mild AD.

DISCUSSION

Correlation of volumetric measures to cognitive change in APOE ε4/ε4 subjects with early AD supports their role as efficacy biomarkers. If confirmed in a Phase 3 trial with ALZ-801 (pro-drug of tramiprosate) in APOE ε4/ε4 early AD subjects, it may allow their use as surrogate outcomes in future treatment or prevention trials in AD.

Current Evidence for a Bidirectional Loop Between the Lysosome and Alpha-Synuclein Proteoforms

Cumulative evidence collected in recent decades suggests that lysosomal dysfunction contributes to neurodegenerative diseases, especially if amyloid proteins are involved. Among these, alpha-synuclein (aSyn) that progressively accumulates and aggregates in Lewy bodies is undisputedly a main culprit in Parkinson disease (PD) pathogenesis. Lysosomal dysfunction is evident in brains of PD patients, and mutations in lysosomal enzymes are a major risk factor of PD. At first glance, the role of protein-degrading lysosomes in a disease with pathological protein accumulation seems obvious and should guide the development of straightforward and rational therapeutic targets. However, our review demonstrates that the story is more complicated for aSyn. The protein can possess diverse posttranslational modifications, aggregate formations, and truncations, all of which contribute to a growing known set of proteoforms. These interfere directly or indirectly with lysosome function, reducing their own degradation, and thereby accelerating the protein aggregation and disease process. Conversely, unbalanced lysosomal enzymatic processes can produce truncated aSyn proteoforms that may be more toxic and prone to aggregation. This highlights the possibility of enhancing lysosomal function as a treatment for PD, if it can be confirmed that this approach effectively reduces harmful aSyn proteoforms and does not produce novel, toxic proteoforms.

Oral Immunization with Soybean Storage Protein Containing Amyloid-β 4-10 Prevents Spatial Learning Decline

 Amyloid-β (Aβ) plays a central role in the pathogenesis of Alzheimer’s disease (AD). Because AD pathologies begin two decades before the onset of dementia, prevention of Aβ amyloidosis has been proposed as a mean to block the pathological cascade. Here, we generate a transgenic plant-based vaccine, a soybean storage protein containing Aβ4–10, named Aβ+, for oral Aβ immunization. One mg of Aβ+ or control protein (Aβ–) was administered to TgCRND8 mice once a week from 9 weeks up to 58 weeks. Aβ+ immunization raised both anti-Aβ antibodies and cellular immune responses. Spatial learning decline was prevented in the Aβ+ immunized group in an extended reference memory version of Morris water maze test from 21 to 57 weeks. In Tris-buffered saline (TBS), sodium dodecyl sulfate (SDS), and formic acid (FA) serial extractions, all sets of Aβ species from Aβ monomer, low to high molecular weight Aβ oligomers, and Aβ smears had different solubility in TgCRND8 brains. Aβ oligomers decreased in TBS fractions, corresponding to an increase in high molecular weight Aβ oligomers in SDS extracts and Aβ smears in FA fraction of the Aβ+ treated group. There was significant inhibition of histological Aβ burden, especially in diffuse plaques, and suppression of microglial inflammation. Processing of amyloid-β protein precursor was not different between Aβ+ and Aβ– groups. No evidence of amyloid-related inflammatory angiopathy was observed. Thus, Aβ+ oral immunization could be a promising, cheap, and long-term safe disease-modifying therapy to prevent the pathological process in AD.

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

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

Aducanumab, gantenerumab, BAN2401, and ALZ-801-the first wave of amyloid-targeting drugs for Alzheimer's disease with potential for near term approval

The body of evidence suggesting a causative, initiating role of beta amyloid (Aβ) in the pathogenesis of Alzheimer’s disease (AD) is substantial. Yet, only a few anti-amyloid agents have shown meaningful efficacy in clinical trials. We evaluated the unifying characteristics of anti-amyloid agents with positive clinical or biomarker effects in long-duration trials and analyzed how pharmacological characteristics determine their clinical product profiles. Four agents with the potential for near term approval fulfill these criteria: the injectable antibodies, aducanumab, gantenerumab, and BAN2401, and a small molecule oral agent, ALZ-801. Aducanumab and BAN2401 showed significant efficacy on both clinical and biomarker outcomes; gantenerumab showed significant biomarker effects, with no clinical efficacy reported to date; and ALZ-801 showed significant clinical effects in the high-risk population of patients homozygous for the ε4 allele of apolipoprotein E gene (APOE4) and a dose-dependent preservation of hippocampal volume. We explored how the pharmacological properties of these agents, namely selectivity for Aβ oligomers, plasma half-life, brain penetration, and time to peak brain exposure, determine their clinical profiles. A crucial characteristic shared by these agents is their ability to engage neurotoxic soluble Aβ oligomers, albeit to various degrees. Aducanumab and gantenerumab partially target oligomers, while mostly clearing insoluble amyloid plaques; BAN2401 preferentially targets soluble protofibrils (large oligomers) over plaques; and ALZ-801 blocks the formation of oligomers without binding to plaques. The degree of selectivity for Aβ oligomers and brain exposure drive the magnitude and onset of clinical efficacy, while the clearance of plaques is associated with vasogenic brain edema. Only the highest doses of aducanumab and BAN2401 show modest efficacy, and higher dosing is limited by increased risk of vasogenic edema, especially in APOE4 carriers. These limitations can be avoided, and efficacy improved by small molecule agents that selectively inhibit the formation or block the toxicity of Aβ oligomers without clearing amyloid plaques. The most advanced selective anti-oligomer agent is ALZ-801, an optimized oral prodrug of tramiprosate, which demonstrated efficacy in homozygous APOE4/4 AD subjects. ALZ-801 selectively and fully inhibits the formation of Aβ42 oligomers at the clinical dose, without evidence of vasogenic edema, and will be evaluated in a phase 3 trial in homozygous APOE4/4 patients with early AD. In addition to clinical measures, the phase 3 trial will include cerebrospinal fluid, plasma, and imaging biomarkers to gain further insights into the role of soluble Aβ oligomers in the pathogenesis of AD and their impact on disease progression.

Passive antiamyloid immunotherapy for Alzheimer's disease

PURPOSE OF REVIEW

Antiamyloid therapy of Alzheimer's disease tackles the overproduction and clearance of the amyloid-beta peptide (Aβ). Immunotherapeutic compounds were tested in large-scale trials. We revisit the recent literature focusing on randomized-controlled trials (RCT) using monoclonal anti-Aβ antibodies.

RECENT FINDINGS

Forty-three articles on anti-Aβ passive immunotherapy for Alzheimer's disease were published between January 2016 and October 2019 regarding 17 RCTs: 13 phase III trials using the monoclonal antibodies bapineuzumab, solanezumab, gantenerumab, crenezumab, and aducanumab; three phase II with crenezumab and aducanumab; and one phase I trial with BAN2401. Studies resulted largely negative considering the effect of the treatment on primary and secondary outcome variables. The incidence of the most important adverse effect, amyloid-related imaging abnormalities (ARIAs) ranged between 0.2 and 22%, in treatment groups. Primary endpoints were not met in eight trials, and five trials were discontinued prior to completion.

SUMMARY

Passive immunotherapy RCTs failed to show clinically relevant effects in patients with clinically manifest or prodromal dementia. The high incidence of ARIAs indicates that the risk of adverse events may outweigh the benefits of these interventions. Ongoing studies must determine the benefit of such interventions in preclinical Alzheimer's disease, addressing the effect of antiamyloid immunotherapy in samples of asymptomatic carriers of autosomal-dominant mutations related to early-onset Alzheimer's disease.

A mechanistic hypothesis for the impairment of synaptic plasticity by soluble Aβ oligomers from Alzheimer's brain

It is increasingly accepted that early cognitive impairment in Alzheimer's disease results in considerable part from synaptic dysfunction caused by the accumulation of a range of oligomeric assemblies of amyloid β-protein (Aβ). Most studies have used synthetic Aβ peptides to explore the mechanisms of memory deficits in rodent models, but recent work suggests that Aβ assemblies isolated from human (AD) brain tissue are far more potent and disease-relevant. Although reductionist experiments show Aβ oligomers to impair synaptic plasticity and neuronal viability, the responsible mechanisms are only partly understood. Glutamatergic receptors, GABAergic receptors, nicotinic receptors, insulin receptors, the cellular prion protein, inflammatory mediators, and diverse signaling pathways have all been suggested. Studies using AD brain-derived soluble Aβ oligomers suggest that only certain bioactive forms (principally small, diffusible oligomers) can disrupt synaptic plasticity, including by binding to plasma membranes and changing excitatory-inhibitory balance, perturbing mGluR, PrP, and other neuronal surface proteins, down-regulating glutamate transporters, causing glutamate spillover, and activating extrasynaptic GluN2B-containing NMDA receptors. We synthesize these emerging data into a mechanistic hypothesis for synaptic failure in Alzheimer's disease that can be modified as new knowledge is added and specific therapeutics are developed.

Oligomerization and Conformational Change Turn Monomeric β-Amyloid and Tau Proteins Toxic: Their Role in Alzheimer's Pathogenesis

The structural polymorphism and the physiological and pathophysiological roles of two important proteins, β-amyloid (Aβ) and tau, that play a key role in Alzheimer's disease (AD) are reviewed. Recent results demonstrate that monomeric Aβ has important physiological functions. Toxic oligomeric Aβ assemblies (AβOs) may play a decisive role in AD pathogenesis. The polymorph fibrillar Aβ (fAβ) form has a very ordered cross-β structure and is assumed to be non-toxic. Tau monomers also have several important physiological actions; however, their oligomerization leads to toxic oligomers (TauOs). Further polymerization results in probably non-toxic fibrillar structures, among others neurofibrillary tangles (NFTs). Their structure was determined by cryo-electron microscopy at atomic level. Both AβOs and TauOs may initiate neurodegenerative processes, and their interactions and crosstalk determine the pathophysiological changes in AD. TauOs (perhaps also AβO) have prionoid character, and they may be responsible for cell-to-cell spreading of the disease. Both extra- and intracellular AβOs and TauOs (and not the previously hypothesized amyloid plaques and NFTs) may represent the novel targets of AD drug research.

A vicious cycle of β amyloid-dependent neuronal hyperactivation

β-amyloid (Aβ)-dependent neuronal hyperactivity is believed to contribute to the circuit dysfunction that characterizes the early stages of Alzheimer's disease (AD). Although experimental evidence in support of this hypothesis continues to accrue, the underlying pathological mechanisms are not well understood. In this experiment, we used mouse models of Aβ-amyloidosis to show that hyperactivation is initiated by the suppression of glutamate reuptake. Hyperactivity occurred in neurons with preexisting baseline activity, whereas inactive neurons were generally resistant to Aβ-mediated hyperactivation. Aβ-containing AD brain extracts and purified Aβ dimers were able to sustain this vicious cycle. Our findings suggest a cellular mechanism of Aβ-dependent neuronal dysfunction that can be active before plaque formation.

Identification of neurotoxic cross-linked amyloid-β dimers in the Alzheimer's brain

The primary structure of canonical amyloid-β-protein was elucidated more than 30 years ago, yet the forms of amyloid-β that play a role in Alzheimer's disease pathogenesis remain poorly defined. Studies of Alzheimer's disease brain extracts suggest that amyloid-β, which migrates on sodium dodecyl sulphate polyacrylamide gel electrophoresis with a molecular weight of ∼7 kDa (7kDa-Aβ), is particularly toxic; however, the nature of this species has been controversial. Using sophisticated mass spectrometry and sensitive assays of disease-relevant toxicity we show that brain-derived bioactive 7kDa-Aβ contains a heterogeneous mixture of covalently cross-linked dimers in the absence of any other detectable proteins. The identification of amyloid-β dimers may open a new phase of Alzheimer's research and allow a better understanding of Alzheimer's disease, and how to monitor and treat this devastating disorder. Future studies investigating the bioactivity of individual dimers cross-linked at known sites will be critical to this effort.

PrP is a central player in toxicity mediated by soluble aggregates of neurodegeneration-causing proteins

Neurodegenerative diseases are an enormous public health problem, affecting tens of millions of people worldwide. Nearly all of these diseases are characterized by oligomerization and fibrillization of neuronal proteins, and there is great interest in therapeutic targeting of these aggregates. Here, we show that soluble aggregates of α-synuclein and tau bind to plate-immobilized PrP in vitro and on mouse cortical neurons, and that this binding requires at least one of the same N-terminal sites at which soluble Aβ aggregates bind. Moreover, soluble aggregates of tau, α-synuclein and Aβ cause both functional (impairment of LTP) and structural (neuritic dystrophy) compromise and these deficits are absent when PrP is ablated, knocked-down, or when neurons are pre-treated with anti-PrP blocking antibodies. Using an all-human experimental paradigm involving: (1) isogenic iPSC-derived neurons expressing or lacking PRNP, and (2) aqueous extracts from brains of individuals who died with Alzheimer's disease, dementia with Lewy bodies, and Pick's disease, we demonstrate that Aβ, α-synuclein and tau are toxic to neurons in a manner that requires PrPC. These results indicate that PrP is likely to play an important role in a variety of late-life neurodegenerative diseases and that therapeutic targeting of PrP, rather than individual disease proteins, may have more benefit for conditions which involve the aggregation of more than one protein.

Beta amyloid aggregates induce sensitised TLR4 signalling causing long-term potentiation deficit and rat neuronal cell death

The molecular events causing memory loss and neuronal cell death in Alzheimer’s disease (AD) over time are still unknown. Here we found that picomolar concentrations of soluble oligomers of synthetic beta amyloid (Aβ42) aggregates incubated with BV2 cells or rat astrocytes caused a sensitised response of Toll-like receptor 4 (TLR4) with time, leading to increased production of TNF-α. Aβ aggregates caused long term potentiation (LTP) deficit in hippocampal slices and predominantly neuronal cell death in co-cultures of astrocytes and neurons, which was blocked by TLR4 antagonists. Soluble Aβ aggregates cause LTP deficit and neuronal death via an autocrine/paracrine mechanism due to TLR4 signalling. These findings suggest that the TLR4-mediated inflammatory response may be a key pathophysiological process in AD.

Hughes et al. investigate the TLR4-mediated inflammatory response in Alzheimer’s disease and show that picomolar concentrations of soluble amyloid beta aggregates lead to a sensitised response of TLR4 with time, resulting in increased TNF-α production. They suggest the use of near physiological concentration of soluble aggregates can cause long-term potentiation deficit and neuronal death through an autocrine/paracrine mechanism due to TLR4 signalling.

The path forward in Alzheimer's disease therapeutics: Reevaluating the amyloid cascade hypothesis

Development of disease-modifying treatments for Alzheimer's disease (AD) has been challenging, with no drugs approved to date. The failures of several amyloid-targeted programs have led many to dismiss the amyloid beta (Aβ) hypothesis of AD. An antiamyloid antibody aducanumab recently showed modest but significant efficacy in a phase 3 trial, providing important validation of amyloid as a therapeutic target. However, the inconsistent results observed with aducanumab may be explained by the limited brain penetration and lack of selectivity for the soluble Aβ oligomers, which are implicated as upstream drivers of neurodegeneration by multiple studies. Development of agents that can effectively inhibit Aβ oligomer formation or block their toxicity is therefore warranted. An ideal drug would cross the blood-brain barrier efficiently and achieve sustained brain levels that can continuously prevent oligomer formation or inhibit their toxicity. A late-stage candidate with these attributes is ALZ-801, an oral drug with a favorable safety profile and high brain penetration that can robustly inhibit Aβ oligomer formation. An upcoming phase 3 trial with ALZ-801 in APOE4/4 homozygous patients with early AD will effectively test this amyloid oligomer hypothesis.

Modeling Alzheimer's disease with iPSC-derived brain cells

Alzheimer's disease is a devastating neurodegenerative disorder with no cure. Countless promising therapeutics have shown efficacy in rodent Alzheimer's disease models yet failed to benefit human patients. While hope remains that earlier intervention with existing therapeutics will improve outcomes, it is becoming increasingly clear that new approaches to understand and combat the pathophysiology of Alzheimer's disease are needed. Human induced pluripotent stem cell (iPSC) technologies have changed the face of preclinical research and iPSC-derived cell types are being utilized to study an array of human conditions, including neurodegenerative disease. All major brain cell types can now be differentiated from iPSCs, while increasingly complex co-culture systems are being developed to facilitate neuroscience research. Many cellular functions perturbed in Alzheimer's disease can be recapitulated using iPSC-derived cells in vitro, and co-culture platforms are beginning to yield insights into the complex interactions that occur between brain cell types during neurodegeneration. Further, iPSC-based systems and genome editing tools will be critical in understanding the roles of the numerous new genes and mutations found to modify Alzheimer's disease risk in the past decade. While still in their relative infancy, these developing iPSC-based technologies hold considerable promise to push forward efforts to combat Alzheimer's disease and other neurodegenerative disorders.

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

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

Target engagement in an alzheimer trial: Crenezumab lowers amyloid β oligomers in cerebrospinal fluid

OBJECTIVE

Oligomeric forms of amyloid β protein (oAβ) are believed to be principally responsible for neurotoxicity in Alzheimer disease (AD), but it is not known whether anti-Aβ antibodies are capable of lowering oAβ levels in humans.

METHODS

We developed an ultrasensitive immunoassay and used it to measure oAβ in cerebrospinal fluid (CSF) from 104 AD subjects participating in the ABBY and BLAZE phase 2 trials of the anti-Aβ antibody crenezumab. Patients received subcutaneous (SC) crenezumab (300mg) or placebo every 2 weeks, or intravenous (IV) crenezumab (15mg/kg) or placebo every 4 weeks for 68 weeks. Ninety-eight of the 104 patients had measurable baseline oAβ levels, and these were compared to levels at week 69 in placebo (n = 28), SC (n = 35), and IV (n = 35) treated patients.

RESULTS

Among those receiving crenezumab, 89% of SC and 86% of IV patients had lower levels of oAβ at week 69 versus baseline. The difference in the proportion of patients with decreasing levels was significant for both treatment arms: p = 0.0035 for SC and p = 0.01 for IV crenezumab versus placebo. The median percentage change was -48% in the SC arm and -43% in the IV arm. No systematic change was observed in the placebo group, with a median change of -13% and equivalent portions with negative and positive change.

INTERPRETATION

Crenezumab lowered CSF oAβ levels in the large majority of treated patients tested. These results support engagement of the principal pathobiological target in AD and identify CSF oAβ as a novel pharmacodynamic biomarker for use in trials of anti-Aβ agents. ANN NEUROL 2019;86:215-224.

A Rationally Designed Humanized Antibody Selective for Amyloid Beta Oligomers in Alzheimer's Disease

Advances in the understanding of Alzheimer’s disease (AD) suggest that pathogenesis is not directly related to plaque burden, but rather to soluble toxic amyloid-beta oligomers (AßO). Therapeutic antibodies targeting Aß monomers and/or plaque have shown limited efficacy and dose-limiting adverse events in clinical trials. These findings suggest that antibodies capable of selectively neutralizing toxic AßO may achieve improved efficacy and safety. To this end, we generated monoclonal antibodies against a conformational Aß epitope predicted by computational modeling to be presented on toxic AßO but not monomers or fibrils. The resulting lead antibody, PMN310, showed the desired AßO-selective binding profile. In vitro, PMN310 inhibited AßO propagation and toxicity. In vivo, PMN310 prevented AßO-induced loss of memory formation and reduced synaptic loss and inflammation. A humanized version (huPMN310) compared favorably to other Aß-directed antibodies showing a lack of adverse event-associated binding to Aß deposits in AD brains, and greater selective binding to AßO-enriched AD brain fractions that contain synaptotoxic Aß species. Systemic administration of huPMN310 in mice resulted in brain exposure and kinetics comparable to those of other therapeutic human monoclonal antibodies. Greater selectivity for AßO and the potential to safely administer high doses of huPMN310 are expected to result in enhanced safety and therapeutic potency.

Alzheimer Disease Pathogenesis: Insights From Molecular and Cellular Biology Studies of Oligomeric Aβ and Tau Species

Alzheimer disease (AD) represents an oncoming epidemic that without an effective treatment promises to exact extraordinary human and financial burdens. Studies of pathogenesis are essential for defining targets for discovering disease-modifying treatments. Past studies of AD neuropathology provided valuable, albeit limited, insights. Nevertheless, building on these findings, recent studies have provided an increasingly rich harvest of genetic, molecular and cellular data that are creating unprecedented opportunities to both understand and treat AD. Among the most significant are those documenting the presence within the AD brain of toxic oligomeric species of Aβ and tau. Existing data support the view that such species can propagate and spread within neural circuits. To place these findings in context we first review the genetics and neuropathology of AD, including AD in Down syndrome (AD-DS). We detail studies that support the existence of toxic oligomeric species while noting the significant unanswered questions concerning their precise structures, the means by which they spread and undergo amplification and how they induce neuronal dysfunction and degeneration. We conclude by offering a speculative synthesis for how oligomers of Aβ and tau initiate and drive pathogenesis. While 100 years after Alzheimer's first report there is much still to learn about pathogenesis and the discovery of disease-modifying treatments, the application of new concepts and sophisticated new tools are poised to deliver important advances for combatting AD.

Aβ(M1-40) and Wild-Type Aβ40 Self-Assemble into Oligomers with Distinct Quaternary Structures

Amyloid-β oligomers (AβOs) self-assemble into polymorphic species with diverse biological activities that are implicated causally to Alzheimer’s disease (AD). Synaptotoxicity of AβO species is dependent on their quaternary structure, however, low-abundance and environmental sensitivity of AβOs in vivo have impeded a thorough assessment of structure–function relationships. We developed a simple biochemical assay to quantify the relative abundance and morphology of cross-linked AβOs. We compared oligomers derived from synthetic Aβ40 (wild-type (WT) Aβ40) and a recombinant source, called Aβ(M1–40). Both peptides assemble into oligomers with common sizes and morphology, however, the predominant quaternary structures of Aβ(M1–40) oligomeric states were more diverse in terms of dispersity and morphology. We identified self-assembly conditions that stabilize high-molecular weight oligomers of Aβ(M1–40) with apparent molecular weights greater than 36 kDa. Given that mixtures of AβOs derived from both peptides have been shown to be potent neurotoxins that disrupt long-term potentiation, we anticipate that the diverse quaternary structures reported for Aβ(M1–40) oligomers using the assays reported here will facilitate research efforts aimed at isolating and identifying common toxic species that contribute to synaptic dysfunction.

PrP-grafted antibodies bind certain amyloid β-protein aggregates, but do not prevent toxicity

BACKGROUND

The prion protein (PrP) is known to bind certain soluble aggregates of the amyloid β-protein (Aβ), and two regions of PrP, one centered around residues 19-33, and the other around 87-112, are thought to be particularly important for this interaction. When either of these sequences are grafted into a human IgG the resulting antibodies react with disease-associated PrP conformers, whereas the parental b12 IgG does not.

METHODS

Human antibodies containing grafts of PrP 19-33 or 87-112 were prepared as before (Solforosi et al., 2007) and tested for their ability to recognize synthetic and Alzheimer's disease (AD) brain-derived Aβ. Since aqueous extracts of AD brain contain a complex mixture of active and inactive Aβ species, we also assessed whether PrP-grafted antibodies could protect against neuritotoxicity mediated by AD brain-derived Aβ. For these experiments, human iPSC-derived neurons were grown in 96-well plates at 5000 cells per well and on post-induction day 21, AD brain extracts were added +/- test antibodies. Neurons were imaged for 3 days using an IncuCyte live-cell imaging system, and neurite number and density quantified.

RESULTS

Grafted antibodies bound a significant portion of aggregated Aβ in aqueous AD extracts, but when these antibodies were co-incubated with neurons treated with brain extracts they did not reduce toxicity. By contrast, the PrP fragment N1 did protect against Aβ.

CONCLUSIONS

These results further demonstrate that not all Aβ oligomers are toxic and suggest that PrP derivatives may allow development of agents that differentially recognize toxic and innocuous Aβ aggregates.

Toxic amyloid-β oligomers induced self-replication in astrocytes triggering neuronal injury

Background

Soluble amyloid-β oligomer (AβO) induced deleterious cascades have recently been considered to be the initiating pathologic agents of Alzheimer's disease (AD). However, little is known about the neurotoxicity and production of different AβOs. Understanding the production and spread of toxic AβOs within the brain is important to improving understanding of AD pathogenesis and treatment.

Methods

Here, PS1V97L transgenic mice, a useful tool for studying the role of AβOs in AD, were used to identify the specific AβO assembly that contributes to neuronal injury and cognitive deficits. Then, we investigated the production and spread of toxic Aβ assemblies in astrocyte and neuron cultures, and further tested the results following intracerebroventricular injection of AβOs in animal model.

Findings

The results showed that cognitive deficits were mainly caused by the accumulation of nonameric and dodecameric Aβ assemblies in the brains. In addition, we found that the toxic AβOs were duplicated in a time-dependent manner when BACE1 and apolipoprotein E were overexpressed, which were responsible for producing redundant Aβ and forming nonameric and dodecameric assemblies in astrocytes, but not in neurons.

Interpretation

Our results suggest that astrocytes may play a central role in the progression of AD by duplicating and spreading toxic AβOs, thus triggering neuronal injury.

Fund

This study was supported by the Key Project of the National Natural Science Foundation of China; the National Key Scientific Instrument and Equipment Development Project; Beijing Scholars Program, and Beijing Brain Initiative from Beijing Municipal Science & Technology Commission.

Amyloid-β immunotherapy for alzheimer disease: Is it now a long shot?

The amyloid-β (Aβ) cascade hypothesis of Alzheimer disease (AD) holds that brain accumulation of Aβ initiates the disease process. Accordingly, drug research has targeted Aβ production, clearance, and deposition as therapeutic strategies. Unfortunately, candidate drugs have failed to show clinical benefit in established, early, or prodromal disease, or in those with high AD risk. Currently, monoclonal antibodies specifically directed against the most neurotoxic Aβ forms are undergoing large-scale trials to confirm initially encouraging results. However, recent findings on the normal physiology of Aβ suggest that accumulation may be compensatory rather than the pathological initiator. If this is true, alternative strategies will be needed to defeat this devastating disease. ANN NEUROL 2019;85:303-315.

Cellular Prion Protein Mediates the Disruption of Hippocampal Synaptic Plasticity by Soluble Tau In Vivo

Intracellular neurofibrillary tangles (NFTs) composed of tau protein are a neuropathological hallmark of several neurodegenerative diseases, the most common of which is Alzheimer's disease (AD). For some time NFTs were considered the primary cause of synaptic dysfunction and neuronal death, however, more recent evidence suggests that soluble aggregates of tau are key drivers of disease. Here we investigated the effect of different tau species on synaptic plasticity in the male rat hippocampus in vivo Intracerebroventricular injection of soluble aggregates formed from either wild-type or P301S human recombinant tau potently inhibited hippocampal long-term potentiation (LTP) at CA3-to-CA1 synapses. In contrast, tau monomers and fibrils appeared inactive. Neither baseline synaptic transmission, paired-pulse facilitation nor burst response during high-frequency conditioning stimulation was affected by the soluble tau aggregates. Similarly, certain AD brain soluble extracts inhibited LTP in a tau-dependent manner that was abrogated by either immunodepletion with, or coinjection of, a mid-region anti-tau monoclonal antibody (mAb), Tau5. Importantly, this tau-mediated block of LTP was prevented by administration of mAbs selective for the prion protein (PrP). Specifically, mAbs to both the mid-region (6D11) and N-terminus (MI-0131) of PrP prevented inhibition of LTP by both recombinant and brain-derived tau. These findings indicate that PrP is a mediator of tau-induced synaptic dysfunction.SIGNIFICANCE STATEMENT Here we report that certain soluble forms of tau selectively disrupt synaptic plasticity in the live rat hippocampus. Further, we show that monoclonal antibodies to cellular prion protein abrogate the impairment of long-term potentiation caused both by recombinant and Alzheimer's disease brain-derived soluble tau. These findings support a critical role for cellular prion protein in the deleterious synaptic actions of extracellular soluble tau in tauopathies, including Alzheimer's disease. Thus, approaches targeting cellular prion protein, or downstream pathways, might provide an effective strategy for developing therapeutics.

A standard model of Alzheimer's disease?

The recent Research Framework proposed by the US National Institute on Aging and the Alzheimer's Association (NIA-AA) recommends that Alzheimer's disease be defined by its specific biology rather than by non-specific neurodegenerative and syndromal features. By affirming markers of abnormal Aβ and tau proteins as the essential pathobiological signature of Alzheimer's disease, the Framework tacitly reinforces the amyloid (Aβ) cascade as the leading theory of Alzheimer pathogenesis. In light of recent evidence that the cascade is driven by the misfolding and templated aggregation of Aβ and tau, we believe that an empirically grounded Standard Model of Alzheimer's pathogenesis is within reach. A Standard Model can clarify and consolidate existing information, contextualize risk factors and the complex disease phenotype, identify testable hypotheses for future research, and pave the most direct path to effective prevention and treatment.

Notch3ECD immunotherapy improves cerebrovascular responses in CADASIL mice

OBJECTIVE

CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy), caused by dominant mutations in the NOTCH3 receptor, is the most aggressive small vessel disease of the brain. A key feature of its pathogenesis is accumulation of the extracellular domain of NOTCH3 receptor (Notch3^ECD ) in small vessels, with formation of characteristic extracellular deposits termed granular osmiophilic material (GOM). Here, we investigated the therapeutic potential of a mouse monoclonal antibody (5E1) that specifically recognizes Notch3^ECD .

METHODS

The binding affinity of 5E1 toward purified NOTCH3 was assessed using Octet analysis. The ability of 5E1 to bind Notch3^ECD deposits in brain vessels and its effects on disease-related phenotypes were evaluated in the CADASIL mouse model, which overexpresses a mutant rat NOTCH3. Notch3^ECD and GOM deposition, white matter lesions, and cerebral blood flow deficits were assessed at treatment initiation (10 weeks) and study completion (30 weeks) using quantitative immunohistochemistry, electron microscopy, and laser-Doppler flowmetry.

RESULTS

5E1 antibody bound recombinant rat NOTCH3 with an average affinity of 317nM. A single peripheral injection of 5E1 robustly decorated Notch3^ECD deposits in the brain vasculature. Chronic administration of 5E1 did not attenuate Notch3^ECD or GOM deposition and was not associated with perivascular microglial activation. It also failed to halt the development of white matter lesions. Despite this, 5E1 treatment markedly protected against impaired cerebral blood flow responses to neural activity and topical application of vasodilators and normalized myogenic responses of cerebral arteries.

INTERPRETATION

This study establishes immunotherapy targeting Notch3^ECD as a new avenue for disease-modifying treatment in CADASIL that warrants further development. Ann Neurol 2018;84:246-259.

Recent advancements toward therapeutic vaccines against Alzheimer's disease

INTRODUCTION

Alzheimer's disease (AD) is a devastating neurodegenerative disease characterized by protein aggregates of amyloid β (Aβ) and tau. These proteins have normal physiological functions, but in AD, they undergo a conformational change and aggregate as toxic oligomeric and fibrillar species with a high β-sheet content.

AREAS COVERED

Active and passive immunotherapeutic approaches are among the most attractive methods for targeting misfolded Aβ and tau. Promising preclinical testing of various immunotherapeutic approaches has yet to translate to cognitive benefits in human clinical trials. Knowledge gained from these past failures has led to the development of second-generation Aβ-active immunotherapies, anti-Aβ monoclonal antibodies targeting a wide array of Aβ conformations, and to a number of immunotherapies targeting pathological tau. This review covers the more recent advances in vaccine development for AD from 2016 to present.

EXPERT COMMENTARY

Due to the complex pathophysiology of AD, greatest clinical efficacy will most likely be achieved by concurrently targeting the most toxic forms of both Aβ and tau.

An in vitro paradigm to assess potential anti-Aβ antibodies for Alzheimer's disease

Although the amyloid β-protein (Aβ) is believed to play an initiating role in Alzheimer's disease (AD), the molecular characteristics of the key pathogenic Aβ forms are not well understood. As a result, it has proved difficult to identify optimal agents that target disease-relevant forms of Aβ. Here, we combined the use of Aβ-rich aqueous extracts of brain samples from AD patients as a source of human Aβ and live-cell imaging of iPSC-derived human neurons to develop a bioassay capable of quantifying the relative protective effects of multiple anti-Aβ antibodies. We report the characterization of 1C22, an aggregate-preferring murine anti-Aβ antibody, which better protects against forms of Aβ oligomers that are toxic to neurites than do the murine precursors of the clinical immunotherapeutics, bapineuzumab and solanezumab. These results suggest further examination of 1C22 is warranted, and that this bioassay maybe useful as a primary screen to identify yet more potent anti-Aβ therapeutics.

Extracellular Forms of Aβ and Tau from iPSC Models of Alzheimer's Disease Disrupt Synaptic Plasticity

The early stages of Alzheimer's disease are associated with synaptic dysfunction prior to overt loss of neurons. To identify extracellular molecules that impair synaptic plasticity in the brain, we studied the secretomes of human iPSC-derived neuronal models of Alzheimer's disease. When introduced into the rat brain, secretomes from human neurons with either a presenilin-1 mutation, amyloid precursor protein duplication, or trisomy of chromosome 21 all strongly inhibit hippocampal long-term potentiation. Synaptic dysfunction caused by presenilin-1 mutant and amyloid precusor protein duplication secretomes is mediated by Aβ peptides, whereas trisomy of chromosome 21 (trisomy 21) neuronal secretomes induce dysfunction through extracellular tau. In all cases, synaptotoxicity is relieved by antibody blockade of cellular prion protein. These data indicate that human models of Alzheimer's disease generate distinct proteins that converge at the level of cellular prion protein to induce synaptic dysfunction in vivo.