Effect of Aβ Oligomers on Neuronal APP Triggers a Vicious Cycle Leading to the Propagation of Synaptic Plasticity Alterations to Healthy Neurons

Rational fusion design inspired by cell-penetrating peptide: SS31/S-14 G Humanin hybrid peptide with amplified multimodal efficacy and bio-permeability for the treatment of Alzheimer's disease

Alzheimer's disease is a neurodegenerative disease induced by multiple interconnected mechanisms. Peptide drug candidates with multi-modal efficacy generated from fusion strategy are suitable for addressing multi-facet pathology. However, clinical translation of peptide drugs is greatly hampered by their low permeability into brain. Herein, a hybrid peptide HNSS is generated by merging two therapeutic peptides (SS31 and S-14 G Humanin (HNG)), using a different approach from the classical shuttle-therapeutic peptide conjugate design. HNSS demonstrated increased bio-permeability, with a 2-fold improvement in brain distribution over HNG, thanks to its structure mimicking the design of signal peptide-derived cell-penetrating peptides. HNSS efficiently alleviated mitochondrial dysfunction through the combined effects of mitochondrial targeting, ROS scavenging and p-STAT3 activation. Meanwhile, HNSS with increased Aβ affinity greatly inhibited Aβ oligomerization/fibrillation, and interrupted Aβ interaction with neuron/microglia by reducing neuronal mitochondrial Aβ deposition and promoting microglial phagocytosis of Aβ. In 3× Tg-AD transgenic mice, HNSS treatment efficiently inhibited brain neuron loss and improved the cognitive performance. This work validates the rational fusion design-based strategy for bio-permeability improvement and efficacy amplification, providing a paradigm for developing therapeutic peptide candidates against neurodegenerative disease.

Firing Alterations of Neurons in Alzheimer's Disease: Are They Merely a Consequence of Pathogenesis or a Pivotal Component of Disease Progression?

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder, yet its underlying causes remain elusive. The conventional perspective on disease pathogenesis attributes alterations in neuronal excitability to molecular changes resulting in synaptic dysfunction. Early hyperexcitability is succeeded by a progressive cessation of electrical activity in neurons, with amyloid beta (Aβ) oligomers and tau protein hyperphosphorylation identified as the initial events leading to hyperactivity. In addition to these key proteins, voltage-gated sodium and potassium channels play a decisive role in the altered electrical properties of neurons in AD. Impaired synaptic function and reduced neuronal plasticity contribute to a vicious cycle, resulting in a reduction in the number of synapses and synaptic proteins, impacting their transportation inside the neuron. An understanding of these neurophysiological alterations, combined with abnormalities in the morphology of brain cells, emerges as a crucial avenue for new treatment investigations. This review aims to delve into the detailed exploration of electrical neuronal alterations observed in different AD models affecting single neurons and neuronal networks.

Amyloid Precursor Protein: A Regulatory Hub in Alzheimer's Disease

Decades of research have demonstrated an incontrovertible role of amyloid-β (Aβ) in the etiology of Alzheimer's disease (AD). However, the overemphasis on the pathological impacts of Aβ may obscure the role of its metabolic precursor, amyloid precursor protein (APP), as a significant hub in the occurrence and progression of AD. The complicated enzymatic processing, ubiquitous receptor-like properties, and abundant expression of APP in the brain, as well as its close links with systemic metabolism, mitochondrial function and neuroinflammation, imply that APP plays multifaceted roles in AD. In this review, we briefly describe the evolutionarily conserved biological characteristics of APP, including its structure, functions and enzymatic processing. We also discuss the possible involvement of APP and its enzymatic metabolites in AD, both detrimental and beneficial. Finally, we describe pharmacological agents or genetic approaches with the capability to reduce APP expression or inhibit its cellular internalization, which can ameliorate multiple aspects of AD pathologies and halt disease progression. These approaches provide a basis for further drug development to combat this terrible disease.

Long term worsening of amyloid pathology, cerebral function, and cognition after a single inoculation of beta-amyloid seeds with Osaka mutation

Alzheimer's disease (AD) is characterized by intracerebral deposition of abnormal proteinaceous assemblies made of amyloid-β (Aß) peptides or tau proteins. These peptides and proteins induce synaptic dysfunctions that are strongly correlated with cognitive decline. Intracerebral infusion of well-defined Aβ seeds from non-mutated Aβ_1-40 or Aβ_1-42 peptides can increase Aβ depositions several months after the infusion. Familial forms of AD are associated with mutations in the amyloid precursor protein (APP) that induce the production of Aβ peptides with different structures. The Aβ Osaka (Aβ_osa mutation (E693Δ)) is located within the Aβ sequence and thus the Aβ_osa peptides have different structures and properties as compared to non-mutated Aβ_1-42 peptides (Aβ_wt). Here, we wondered if a single exposure to this mutated Aβ can worsen AD pathology as well as downstream events including cognition, cerebral connectivity and synaptic health several months after the inoculation. To answer this question we inoculated Aβ_1-42-bearing Osaka mutation (Aβ_osa) in the dentate gyrus of APP_swe/PS1_dE9 mice at the age of two months. Their cognition and cerebral connectivity were analyzed at 4 months post-inoculation by behavioral evaluation and functional MRI. Aβ pathology as well as synaptic density were evaluated by histology. The impact of Aβ_osa peptides on synaptic health was also measured on primary cortical neurons. Remarkably, the intracerebral administration of Aβ_osa induced cognitive and synaptic impairments as well as a reduction of functional connectivity between different brain regions, 4 months post-inoculation. It increased Aβ plaque depositions and increased Aβ oligomers. This is the first study showing that a single, sporadic event as Aβ_osa inoculation can worsen the fate of the pathology and clinical outcome several months after the event. It suggests that a single inoculation of Aβ regulates a large cascade of events for a long time.

Therapeutic potential of natural molecules against Alzheimer's disease via SIRT1 modulation

Alzheimer's disease (AD) is a neurodegenerative disease mainly characterized by progressive cognitive dysfunction and memory impairment. Recent studies have shown that regulating silent information regulator 1 (SIRT1) expression has a significant neuroprotective effect, and SIRT1 may become a new therapeutic target for AD. Natural molecules are an important source of drug development for use in AD therapy and may regulate a wide range of biological events by regulating SIRT1 as well as other SIRT1-mediated signaling pathways. This review aims to summarize the correlation between SIRT1 and AD and to identify in vivo and in vitro studies investigating the anti-AD properties of natural molecules as modulators of SIRT1 and SIRT1-mediated signaling pathways. A literature search was conducted for studies published between January 2000 and October 2022 using various literature databases, including Web of Science, PubMed, Google Scholar, Science Direct, and EMBASE. Natural molecules, such as resveratrol, quercetin, icariin, bisdemethoxycurcumin, dihydromyricetin, salidroside, patchouli, sesamin, rhein, ligustilide, tetramethoxyflavanone, 1-theanine, schisandrin, curcumin, betaine, pterostilbene, ampelopsin, schisanhenol, and eriodictyol, have the potential to modulate SIRT1 and SIRT1 signaling pathways, thereby combating AD. The natural molecules modulating SIRT1 discussed in this review provide a potentially novel multi-mechanistic therapeutic strategy for AD. However, future clinical trials need to be conducted to further investigate their beneficial properties and to determine the safety and efficacy of SIRT1 natural activators against AD.

Aβ42 oligomer-specific antibody ALZ-201 reduces the neurotoxicity of Alzheimer's disease brain extracts

BACKGROUND

In Alzheimer's disease (AD), amyloid-β 1-42 (Aβ42) neurotoxicity stems mostly from its soluble oligomeric aggregates. Studies of such aggregates have been hampered by the lack of oligomer-specific research tools and their intrinsic instability and heterogeneity. Here, we developed a monoclonal antibody with a unique oligomer-specific binding profile (ALZ-201) using oligomer-stabilising technology. Subsequently, we assessed the etiological relevance of the Aβ targeted by ALZ-201 on physiologically derived, toxic Aβ using extracts from post-mortem brains of AD patients and controls in primary mouse neuron cultures.

METHODS

Mice were immunised with stable oligomers derived from the Aβ42 peptide with A21C/A30C mutations (AβCC), and ALZ-201 was developed using hybridoma technology. Specificity for the oligomeric form of the Aβ42CC antigen and Aβ42 was confirmed using ELISA, and non-reactivity against plaques by immunohistochemistry (IHC). The antibody's potential for cross-protective activity against pathological Aβ was evaluated in brain tissue samples from 10 individuals confirmed as AD (n=7) and non-AD (n=3) with IHC staining for Aβ and phosphorylated tau (p-Tau) aggregates. Brain extracts were prepared and immunodepleted using the positive control 4G8 antibody, ALZ-201 or an isotype control to ALZ-201. Fractions were biochemically characterised, and toxicity assays were performed in primary mouse neuronal cultures using automated high-content microscopy.

RESULTS

AD brain extracts proved to be more toxic than controls as demonstrated by neuronal loss and morphological determinants (e.g. synapse density and measures of neurite complexity). Immunodepletion using 4G8 reduced Aβ levels in both AD and control samples compared to ALZ-201 or the isotype control, which showed no significant difference. Importantly, despite the differential effect on the total Aβ content, the neuroprotective effects of 4G8 and ALZ-201 immunodepletion were similar, whereas the isotype control showed no effect.

CONCLUSIONS

ALZ-201 depletes a toxic species in post-mortem AD brain extracts causing a positive physiological and protective impact on the integrity and morphology of mouse neurons. Its unique specificity indicates that a low-abundant, soluble Aβ42 oligomer may account for much of the neurotoxicity in AD. This critical attribute identifies the potential of ALZ-201 as a novel drug candidate for achieving a true, clinical therapeutic effect in AD.

Neurexin-β Mediates the Synaptogenic Activity of Amyloid Precursor Protein

In addition to its role in Alzheimer's disease, amyloid precursor protein (APP) has physiological roles in synapse development and function. APP induces presynaptic differentiation when presented to axons, but the mechanism is unknown. Here we show that APP binds neurexin to mediate this synaptogenic activity. APP specifically binds β not α neurexins modulated by splice site 4. Binding to neurexin heparan sulfate glycan and LNS protein domains is required for high-affinity interaction and for full-length APP to recruit axonal neurexin. The synaptogenic activity of APP is abolished by triple knockdown of neurexins in hippocampal neurons pooled from male and female rats. Based on these and previous results, our model is that a dendritic-axonal trans dimer of full-length APP binds to axonal neurexin-β to promote synaptic differentiation and function. Furthermore, soluble sAPPs also bind neurexin-β and inhibit its interaction with neuroligin-1, raising the possibility that disruption of neurexin function by altered levels of full-length APP and its cleavage products may contribute to early synaptic deficits in Alzheimer's disease.SIGNIFICANCE STATEMENT The prevailing model for the basis of Alzheimer's disease is the amyloid cascade triggered by altered cleavage of amyloid precursor protein (APP). APP also has physiological roles at the synapse, but the molecular basis for its synaptic functions is not well understood. Here, we show that APP binds the presynaptic organizing protein neurexin-β and that neurexin is essential for the synaptogenic activity of APP. Furthermore, soluble APP forms generated by APP cleavage also bind neurexin-β and can block interaction with transmembrane synaptogenic ligands of neurexin. These findings reveal a role for neurexin-APP interaction in synapse development and raise the possibility that disruptions of neurexin function may contribute to synaptic and cognitive deficits in the critical early stage of Alzheimer's disease.

Evaluation of Synaptic and Axonal Dysfunction Biomarkers in Alzheimer's Disease and Mild Cognitive Impairment Based on CSF and Bioinformatic Analysis

Synaptic loss and dysfunction are one of the earliest signs of neurodegeneration associated with cognitive decline in Alzheimer's disease (AD) and other neurodegenerative diseases. This study aimed to assess the relationships between biological processes of the synaptic pathology underlying AD, molecular functions, and dynamics of the change concentrations of selected proteins reflecting synaptic and axonal pathology in dementia stages. Neurogranin (Ng), neuronal pentraxin receptor (NPTXR), and Visinin-like protein 1 (VILIP1) concentrations were measured in the cerebrospinal fluid (CSF) of MCI, AD, and non-demented controls (CTRL) using quantitative immunological methods. Gene ontology (GO) enrichment analysis was used for the functional analysis of tested proteins. The CSF Aβ42/Ng ratio was significantly different between all the compared groups. The CSF NPTXR/Ng ratio was significantly different between MCI compared to CTRL and AD compared to CTRL. The GO enrichment analysis revealed that two terms (the Biological Process (BP) and Cellular Component (CC) levels) are significantly enriched for NPTXR and Ng but not for VILIP1. Both Ng and NPTXR concentrations in CSF are promising synaptic dysfunction biomarkers for the early diagnosis of the disease. Moreover, both proteins are biochemically associated with classical biomarkers and VILIP-1. Mapping shared molecular and biological functions for the tested proteins by GO enrichment analysis may be beneficial in screening and setting new research targets.

A novel non‑selective atypical PKC agonist could protect neuronal cell line from Aβ‑oligomer induced toxicity by suppressing Aβ generation

Atypical protein kinase C (aPKCs) serve key functions in embryonic development by regulating apical-basal polarity. Previous studies have shed light on their roles during adulthood, especially in the development of Alzheimer's disease (AD). Although the crystal structure of PKCι has been resolved, an agonist of aPKCs remains to be discovered. In the present study, by using the Discovery Studio program and LibDock methodology, a small molecule library (K66-X4436 KINA Set) of compounds were screened for potential binding to PKCι. Subsequently, the computational docking results were validated using affinity selection-mass spectrometry, before in vitro kinase activity was used to determine the function of the hit compounds. A cell-based model assay that can mimic the pathology of AD was then established and used to assess the function of these hit compounds. As a result, the aPKC agonist Z640 was identified, which could bind to PKCι in silico, in vitro and in this cell-based model. Z640 was further confirmed as a non-selective aPKC agonist that can activate the kinase activity of both PKCι and PKCζ. In the cell-based assay, Z640 was found to protect neuronal cell lines from amyloid-β (Aβ) oligomer-induced cell death by reducing reactive oxygen species production and restore mitochondrial function. In addition, Z640 could reduce Aβ40 generation in a dose-dependent manner and shift amyloid precursor protein processing towards the non-amyloid pathway. To conclude, the present study is the first, to the best of the authors' knowledge to identify an aPKC agonist by combining computer-assisted drug discovery and cell-based assays. The present study also revealed that aPKC agonists have therapeutic potential for the treatment of AD.

Increased P2×2 receptors induced by amyloid-β peptide participates in the neurotoxicity in alzheimer's disease

Amyloid beta peptide (Aβ) is tightly associated with the physiopathology of Alzheimer's Disease (AD) as one of the most important factors in the evolution of the pathology. In this context, we previously reported that Aβ increases the expression of ionotropic purinergic receptor 2 (P2×2R). However, its role on the cellular and molecular Aβ toxicity is unknown, especially in human brain of AD patients. Using cellular and molecular approaches in hippocampal neurons, PC12 cells, and human brain samples of patients with AD, we evaluated the participation of P2×2R in the physiopathology of AD. Here, we reported that Aβ oligomers (Aβo) increased P2×2 levels in mice hippocampal neurons, and that this receptor increases at late Braak stages of AD patients. Aβo also increases the colocalization of APP with Rab5, an early endosomes marker, and decreased the nuclear/cytoplasmic ratio of Fe65 and PGC-1α immunoreactivity. The overexpression in PC12 cells of P2×2a, but not P2×2b, replicated these changes in Fe65 and PGC-1α; however, both overexpressed isoforms increased levels of Aβ. Taken together, these data suggest that P2×2 is upregulated in AD and it could be a key potentiator of the physiopathology of Aβ. Our results point to a possible participation in a toxic cycle that increases Aβ production, Ca²⁺ overload, and a decrease of PGC-1α. These novel findings put the P2×2R as a key novel pharmacological target to develop new therapeutic strategies to treat Alzheimer's Disease.

Insulin Resistance Exacerbates Alzheimer Disease via Multiple Mechanisms

Alzheimer disease (AD) is a chronic neurodegenerative disease that accounts for 60–70% of dementia and is the sixth leading cause of death in the United States. The pathogenesis of this debilitating disorder is still not completely understood. New insights into the pathogenesis of AD are needed in order to develop novel pharmacologic approaches. In recent years, numerous studies have shown that insulin resistance plays a significant role in the development of AD. Over 80% of patients with AD have type II diabetes (T2DM) or abnormal serum glucose, suggesting that the pathogenic mechanisms of insulin resistance and AD likely overlap. Insulin resistance increases neuroinflammation, which promotes both amyloid β-protein deposition and aberrant tau phosphorylation. By increasing production of reactive oxygen species, insulin resistance triggers amyloid β-protein accumulation. Oxidative stress associated with insulin resistance also dysregulates glycogen synthase kinase 3-β (GSK-3β), which leads to increased tau phosphorylation. Both insulin and amyloid β-protein are metabolized by insulin degrading enzyme (IDE). Defects in this enzyme are the basis for a strong association between T2DM and AD. This review highlights multiple pathogenic mechanisms induced by insulin resistance that are implicated in AD. Several pharmacologic approaches to AD associated with insulin resistance are presented.

The plasminogen activating system in the pathogenesis of Alzheimer's disease

Dementia is a clinical syndrome that affects approximately 47 million people worldwide and is characterized by progressive and irreversible decline of cognitive, behavioral and sesorimotor functions. Alzheimer's disease (AD) accounts for approximately 60-80% of all cases of dementia, and neuropathologically is characterized by extracellular deposits of insoluble amyloid-β (Aβ) and intracellular aggregates of hyperphosphorylated tau. Significantly, although for a long time it was believed that the extracellular accumulation of Aβ was the culprit of the symptoms observed in these patients, more recent studies have shown that cognitive decline in people suffering this disease is associated with soluble Aβ-induced synaptic dysfunction instead of the formation of insoluble Aβ-containing extracellular plaques. These observations are translationally relevant because soluble Aβ-induced synaptic dysfunction is an early event in AD that precedes neuronal death, and thus is amenable to therapeutic interventions to prevent cognitive decline before the progression to irreversible brain damage. The plasminogen activating (PA) system is an enzymatic cascade that triggers the degradation of fibrin by catalyzing the conversion of plasminogen into plasmin via two serine proteinases: tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). Experimental evidence reported over the last three decades has shown that tPA and uPA play a role in the pathogenesis of AD. However, these studies have focused on the ability of these plasminogen activators to trigger plasmin-induced cleavage of insoluble Aβ-containing extracellular plaques. In contrast, recent evidence indicates that activity-dependent release of uPA from the presynaptic terminal of cerebral cortical neurons protects the synapse from the deleterious effects of soluble Aβ via a mechanism that does not require plasmin generation or the cleavage of Aβ fibrils. Below we discuss the role of the PA system in the pathogenesis of AD and the translational relevance of data published to this date.