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Introduction Alzheimer’s disease brains are characterized by extracellular plaques containing the aggregated amyloid β 42 (Aβ 42 ) peptide and intraneuronal tangles containing hyperphosphorylated tau. Aβ 42 is produced by sequential processing of the amyloid precursor protein (APP) by β-secretase followed by γ-secretase. Substantial efforts have been put into developing pharmaceuticals preventing the production or increasing the clearance of Aβ 42 . However, treatments inhibiting γ-secretase have proven disappointing due to off-target effects. To circumvent these effects, γ-secretase modulators (GSMs) have been developed, which rather than inhibiting γ-secretase shift its preference into producing less aggregation-prone shorter Aβ peptides. Belonging to the same family of proteins as APP, amyloid-like protein 1 (APLP1) is also a substrate for γ-secretase. Herein we investigated whether the GSM E2012 affects APLP1 processing in the central nervous system by measuring APLP1 peptide levels in cerebrospinal fluid (CSF) before and after E2012 treatment in dogs. Methods An in-house monoclonal APLP1 antibody, AP1, was produced and utilized for immunopurification of APLP1 from human and dog CSF in a hybrid immuno-affinity mass spectrometric method. Seven dogs received a single dose of 20 or 80 mg/kg of E2012 in a randomized cross-over design and CSF was collected prior to and 4, 8 and 24 hours after dosing. Results We have identified 14 CSF APLP1 peptides in humans and 12 CSF APLP1 peptides in dogs. Of these, seven were reproducibly detectable in dogs who received E2012. We found a dose-dependent relative increase of the CSF peptides APLP1β17, 1β18 and 1β28 accompanied with a decrease of 1β25 and 1β27 in response to E2012 treatment. All peptides reverted to baseline over the time of sample collection. Conclusion We show an in vivo effect of the GSM E2012 on the processing of APLP1 which is measurable in CSF. These data suggest that APLP1 peptides may be used as biomarkers to monitor drug effects of GSMs on γ-secretase processing in clinical trials. However, this requires further investigation in larger cohorts, including studies in man.

The characterization of the C-terminal intracellular region of the amyloid precursor protein (APP) has been challenging due to its intrinsically disordered nature. Unlike the E1 and E2 subdomains, which show well-defined folded structures (Figure 1A), the C-terminal region does not adopt a fixed conformation under normal physiological conditions. Furthermore, while this domain can adopt specific folds upon binding to certain interactors, its structural flexibility and lack of complete structural information complicates the development of specific antibodies against this region. Here, we generated and characterized a novel monoclonal antibody (APP1B) against the C-terminal region of APP outside of the Ab region and performed structural computational analyses of the antibody. The antibody, antigen, and antibody-antigen structures were predicted using AlphaFold2.3 and AlphaFold2.3 Multimer. The obtained structures were used to predict the specificity of the developed antibody against the target peptide (by comparing the confidence scores against the other soluble peptide fragments of the APP protein), and computationally predict its binding affinity against the antigen using Protein Energy Landscape Exploration (PELE) and Molecular Dynamics (MD). The antigen adopted a fixed, high-confidence structure in the presence of the antibody, revealing a clear binding site (Figure 1B). Using PELE, we identified an energetic minimum at ∼6 Å from the initial antigen-antibody conformation, with the antibody's interacting region remaining stable. The solvent-accessible surface area analysis indicated reduced solvent accessibility for the antigen due to antibody binding (Figure 1C). MD simulations showed high RMSD variability in the antibody's heavy chain CDR3, while the antigen's targeted region exhibited lower, stable RMSD than the whole antigen, confirming interaction with the antibody paratope (Figure 1D-F). Specificity analysis using AlphaFold2 Multimer revealed the highest predicted alignment error (pAE) at the APP1B-target interface compared to other APP fragments, and no significant interactions with APLP1 or APLP2, confirming antibody specificity. Our computational and structural analyses validate APP1B's high-affinity interaction with the target antigen (APP C-terminal region). This specificity, coupled with the lack of cross-reactivity with APLP1 or APLP2, positions APP1B as a promising candidate for targeted research.

The profile of autoantibodies is dysregulated in patients with Alzheimer’s disease (AD). Autoantibodies to beta-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1) are present in human blood. This study aims to investigate the clinical relevance and pathophysiological roles of autoantibodies to BACE1 in AD. Clinical investigations were conducted in two independent cohorts, the Chongqing cohort, and the Australian Imaging, Biomarkers, and Lifestyle (AIBL) cohort. The Chongqing cohort included 55 AD patients, 28 patients with non-AD dementia, and 70 cognitively normal subjects (CN). The AIBL cohort included 162 Aβ-PET − CN, 169 Aβ-PET + cognitively normal subjects (preclinical AD), and 31 Aβ-PET + cognitively impaired subjects (Clinical AD). Plasma autoantibodies to BACE1 were determined by one-site Elisa. The associations of plasma autoantibodies to BACE1 with brain Aβ load and cognitive trajectory were investigated. The effects of autoantibodies to BACE1 on AD-type pathologies and underlying mechanisms were investigated in APP/PS1 mice and SH/APPswe/PS1wt cell lines. In the Chongqing cohort, plasma autoantibodies to BACE1 were higher in AD patients, in comparison with CN and non-AD dementia patients. In the AIBL cohort, plasma autoantibodies to BACE1 were highest in clinical AD patients, followed by preclinical AD and CN subjects. Higher autoantibodies to BACE1 were associated with an increased incidence of brain amyloid positivity conversion during follow-up. Autoantibodies to BACE1 exacerbated brain amyloid deposition and subsequent AD-type pathologies, including Tau hyperphosphorylation, neuroinflammation, and neurodegeneration in APP/PS1 mice. Autoantibodies to BACE1 increased Aβ production by promoting BACE1 expression through inhibiting PPARγ signaling. These findings suggest that autoantibodies to BACE1 are pathogenic in AD and the upregulation of these autoantibodies may promote the development of the disease. This study offers new insights into the mechanism of AD from an autoimmune perspective.

Generation of neurotoxic amyloid β peptides and their deposition along with neurofibrillary tangle formation represent key pathological hallmarks in Alzheimer’s disease (AD). Recent evidence suggests that inflammation may be a third important component which, once initiated in response to neurodegeneration or dysfunction, may actively contribute to disease progression and chronicity. Various neuroinflammatory mediators including complement activators and inhibitors, chemokines, cytokines, radical oxygen species and inflammatory enzyme systems are expressed and released by microglia, astrocytes and neurons in the AD brain. Degeneration of aminergic brain stem nuclei including the locus ceruleus and the nucleus basalis of Meynert may facilitate the occurrence of inflammation in their projection areas given the antiinflammatory and neuroprotective action of their key transmitters norepinephrine and acetylcholine. While inflammation has been thought to arise secondary to degeneration, recent experiments demonstrated that inflammatory mediators may stimulate amyloid precursor protein processing by various means and therefore can establish a vicious cycle. Despite the fact that some aspects of inflammation may even be protective for bystander neurons, antiinflammatory treatment strategies should therefore be considered. Non-steroidal anti-inflammatory drugs have been shown to reduce the risk and delay the onset to develop AD. While, the precise molecular mechanism underlying this effect is still unknown, a number of possible mechanisms including cyclooxygenase 2 or γ-secretase inhibition and activation of the peroxisome proliferator activated receptor γ may alone or, more likely, in concert account for the epidemiologically observed protection.

Some studies have described that Amyloid Precursor Protein (APP) C-terminal fragments (CTFs) accumulate in the brain of patients with Alzheimer's disease (AD). These data rely mostly on biochemical techniques, but the neuroanatomical distribution in human AD brain remains unknown. In this work, we generated a novel APP antibody to investigate the morphological distribution of APP-CTF accumulation in postmortem human brain. Cross-sectional neuropathological study. Formalin-fixed paraffin-embedded (FFPE) post-mortem human brain samples from the hippocampus, frontal and temporal cortex were obtained from the Neurological Tissue Bank (IDIBAPS-Hospital Clinic Barcelona). The study group consisted of 48 individuals: 10 cases with neuropathological criteria of early-onset SAD, 12 with ADAD (8 PSEN1, 2 APP mutation carriers, 2 APP duplications), 5 with DS, 10 with DS-AD, 7 controls without neurodegenerative pathology and 4 individuals with other neurodegenerative diseases (1 sFTLD-TDP, 1 C9orf72-FTLD-TDP, 1 DLB and 1 ALS-TDP). Antibody generation: APP1B was obtained from 8-10-week female BALB/c mice immunized with a C-terminal APP peptide. Selected hybridomas were cloned and stable clones producing antibodies were expanded. The antibodies were purified using protein G affinity chromatography. Cell culture: Human fetal tissue was obtained to generate primary human cortical culture derived from DS fetuses. A commercially available stable cell line expressing APP-C99-GFP was used for immunocytochemistry experiments. We generated and characterized a novel murine antibody (APP1B) against the C-terminus of APP. APP1B showed specificity for APP-CTF in primary human cortical cell culture obtained from DS fetuses and a cell line expressing human APP-C99. Immunohistochemistry experiments with APP1B antibody in human brain revealed extensive intracellular \"tangle-like\" neuronal APP (iAPP) immunoreactivities in AD-vulnerable areas in sporadic, autosomal dominant AD and in Down syndrome with AD (Figure 1). The immunoreactivity pattern was absent in controls without pathology, DS without AD or other neurodegenerative conditions. iAPP pathology did not colocalize with tau or ubiquitin but followed the neuroanatomical distribution of neuronal degeneration in AD (Figure 2). We propose iAPP pathology as an unrecognized core neuropathological hallmark of AD. Due to the topographical distribution and selective neuronal involvement, iAPP pathology may represent a link between APP dyshomeostasis and progressive tau aggregation.

The mechanisms of amyloid-β (Aβ)-degradation and clearance in Alzheimer’s disease (AD) pathogenesis have been relatively little studied. Short Aβ-fragments form by enzymatic cleavage and alternate amyloid-beta precursor protein (APP)-processing. Here we characterized a novel polyclonal Aβ-antibody raised against an Aβ mid-domain and used it to investigate microglial Aβ-uptake in situ by microscopy at the light- and ultrastructural levels. The rabbit Aβ-mid-domain antibody (ab338), raised against the mid-domain amino acids 21–34 (Aβ 21–34 ), was characterized with biochemical and histological techniques. To identify the epitope in Aβ recognized by ab338, solid phase and solution binding data were compared with peptide folding scores as calculated with the Tango software. The ab338 antibody displayed high average affinity (K D : 6.2 × 10 −10  M) and showed preference for C-terminal truncated Aβ-peptides ending at amino acid 34 and Aβ-mid domain peptides with high scores of β-turn structure. In transgenic APP-mouse brain, ab338 labelled amyloid plaques and detected Aβ-fragments in microglia at the ultra- and light microscopic levels. This reinforces a role of microglia/macrophages in Aβ-clearance in vivo. The ab338 antibody might be a valuable tool to study Aβ-clearance by microglial uptake and Aβ-mid-domain peptides generated by enzymatic degradation and alternate production.

In Alzheimer's disease (AD), one of the early responses to Aβ amyloidosis is recruitment of microglia to areas of new plaque. Microglial receptors such as cannabinoid receptor 2 (CB2) might be a suitable target for development of PET radiotracers that could serve as imaging biomarkers of Aβ-induced neuroinflammation. Mouse models of amyloidosis (J20APPswe/ind and APPswe/PS1ΔE9) were used to investigate the cellular distribution of CB2 receptors. Specificity of CB2 antibody (H60) was confirmed using J20APPswe/ind mice lacking CB2 receptors. APPswe/PS1ΔE9 mice were used in small animal PET with a CB2-targeting radiotracer, [11C]A836339. These studies revealed increased binding of [11C]A836339 in amyloid-bearing mice. Specificity of the PET signal was confirmed in a blockade study with a specific CB2 antagonist, AM630. Confocal microscopy revealed that CB2-receptor immunoreactivity was associated with astroglial (GFAP) and, predominantly, microglial (CD68) markers. CB2 receptors were observed, in particular, in microglial processes forming engulfment synapses with Aβ plaques. In contrast to glial cells, neuron (NeuN)-derived CB2 signal was equal between amyloid-bearing and control mice. The pattern of neuronal CB2 staining in amyloid-bearing mice was similar to that in human cases of AD. The data collected in this study indicate that Aβ amyloidosis without concomitant tau pathology is sufficient to activate CB2 receptors that are suitable as an imaging biomarker of neuroinflammation. The main source of enhanced CB2 PET binding in amyloid-bearing mice is increased CB2 immunoreactivity in activated microglia. The presence of CB2 immunoreactivity in neurons does not likely contribute to the enhanced CB2 PET signal in amyloid-bearing mice due to a lack of significant neuronal loss in this model. However, significant loss of neurons as seen at late stages of AD might decrease the CB2 PET signal due to loss of neuronally-derived CB2. Thus this study in mouse models of AD indicates that a CB2-specific radiotracer can be used as a biomarker of neuroinflammation in the early preclinical stages of AD, when no significant neuronal loss has yet developed.

This study identified 13 endoplasmic reticulum stress (ERS)-related biomarkers associated with multiple sclerosis (MS) through integrated bioinformatics analysis (including weighted gene co-expression network analysis and machine learning algorithms) and single-cell sequencing, combined with validation in an experimental autoimmune encephalomyelitis (EAE) mouse model. Among them, GPX1, RCN1, and UBE2D3 exhibited high diagnostic value (AUC > 0.7, p < 0.05), and the diagnostic potential of GPX1 and RCN1 was confirmed in the animal model. The study found that memory B cells, plasma cells, neutrophils, and M1 macrophages were significantly increased in MS patients, while naive B cells and activated NK cells decreased. Consensus clustering based on key ERS-related genes divided MS patients into two subtypes. Single-cell sequencing showed that microglia and pericytes were the cell types with the highest expression of key ERS-related genes, and the APP-CD74 pathway was enhanced in the brain tissue of MS patients. Mendelian randomization analysis suggested that GPX1 plays a protective role in MS. These findings reveal the mechanisms of ERS-related biomarkers in MS and provide potential targets for diagnosis and treatment.

It is well known that mutations in the gene coding for amyloid precursor protein are responsible for autosomal dominant forms of Alzheimer’s disease. Proteolytic processing of the protein leads to a number of metabolites including the amyloid beta peptide. Although brain amyloid precursor protein expression and amyloid beta production are associated with the pathophysiology of Alzheimer’s disease, it is clear that amyloid precursor protein is expressed in numerous cell types and tissues. Here we demonstrate that amyloid precursor protein is involved in regulating the phenotype of both adipocytes and peripheral macrophages and is required for high fat diet-dependent weight gain in mice. These data suggest that functions of this protein include modulation of the peripheral immune system and lipid metabolism. This biology may have relevance not only to the pathophysiology of Alzheimer’s disease but also diet-associated obesity.

Background The form(s) of amyloid-β peptide (Aβ) associated with the pathology characteristic of Alzheimer's disease (AD) remains unclear. In particular, the neurotoxicity of intraneuronal Aβ accumulation is an issue of considerable controversy; even the existence of Aβ deposits within neurons has recently been challenged by Winton and co-workers. These authors purport that it is actually intraneuronal APP that is being detected by antibodies thought to be specific for Aβ. To further address this issue, an anti-Aβ antibody was developed (MOAB-2) that specifically detects Aβ, but not APP. This antibody allows for the further evaluation of the early accumulation of intraneuronal Aβ in transgenic mice with increased levels of human Aβ in 5xFAD and 3xTg mice. Results MOAB-2 (mouse IgG 2b ) is a pan-specific, high-titer antibody to Aβ residues 1-4 as demonstrated by biochemical and immunohistochemical analyses (IHC), particularly compared to 6E10 (a commonly used commercial antibody to Aβ residues 3-8). MOAB-2 did not detect APP or APP-CTFs in cell culture media/lysates (HEK-APP Swe or HEK-APP Swe /BACE1) or in brain homogenates from transgenic mice expressing 5 familial AD (FAD) mutation (5xFAD mice). Using IHC on 5xFAD brain tissue, MOAB-2 immunoreactivity co-localized with C-terminal antibodies specific for Aβ40 and Aβ42. MOAB-2 did not co-localize with either N- or C-terminal antibodies to APP. In addition, no MOAB-2-immunreactivity was observed in the brains of 5xFAD/BACE -/- mice, although significant amounts of APP were detected by N- and C-terminal antibodies to APP, as well as by 6E10. In both 5xFAD and 3xTg mouse brain tissue, MOAB-2 co-localized with cathepsin-D, a marker for acidic organelles, further evidence for intraneuronal Aβ, distinct from Aβ associated with the cell membrane. MOAB-2 demonstrated strong intraneuronal and extra-cellular immunoreactivity in 5xFAD and 3xTg mouse brain tissues. Conclusions Both intraneuronal Aβ accumulation and extracellular Aβ deposition was demonstrated in 5xFAD mice and 3xTg mice with MOAB-2, an antibody that will help differentiate intracellular Aβ from APP. However, further investigation is required to determine whether a molecular mechanism links the presence of intraneuronal Aβ with neurotoxicity. As well, understanding the relevance of these observations to human AD patients is critical.