η-Secretase processing of APP inhibits neuronal activity in the hippocampus

Eta-secretase-like processing of the amyloid precursor protein (APP) by the rhomboid protease RHBDL4

The amyloid precursor protein (APP) is a key protein in Alzheimer's disease synthesized in the endoplasmic reticulum (ER) and translocated to the plasma membrane where it undergoes proteolytic cleavages by several proteases. Conversely to other known proteases, we previously elucidated rhomboid protease RHBDL4 as a novel APP processing enzyme where several cleavages likely occur already in the ER. Interestingly, the pattern of RHBDL4-derived large APP C-terminal fragments resemble those generated by the η-secretase or MT5-MMP, which was described to generate so called Aη fragments. The similarity in large APP C-terminal fragments between both proteases raised the question whether RHBDL4 may contribute to η-secretase activity and Aη-like fragments. Here, we identified two cleavage sites of RHBDL4 in APP by mass spectrometry, which, intriguingly, lie in close proximity to the MT5-MMP cleavage sites. Indeed, we observed that RHBDL4 generates Aη-like fragments in vitro without contributions of α-, β-, or γ-secretases. Such Aη-like fragments are likely generated in the ER since RHBDL4-derived APP-C-terminal fragments do not reach the cell surface. Inherited, familial APP mutations appear to not affect this processing pathway. In RHBDL4 knockout mice, we observed increased cerebral full length APP in comparison to wild type (WT) in support of RHBDL4 being a physiologically relevant protease for APP. Furthermore, we found secreted Aη fragments in dissociated mixed cortical cultures from WT mice, however significantly less Aη fragments in RHBDL4 knockout cultures. Our data underscores that RHBDL4 contributes to η-secretease-like processing of APP and that RHBDL4 is a physiologically relevant protease for APP.

Evaluating the Role of N-Acetyl-L-Tryptophan in the Aβ 1-42-Induced Neuroinflammation and Cognitive Decline in Alzheimer's Disease

Alzheimer's disease (AD), a neurodegenerative condition previously known to affect the older population, is also now seen in younger individuals. AD is often associated with cognitive decline and neuroinflammation elevation primarily due to amyloid β (Aβ) accumulation. Multiple pathological complications in AD call for therapies with a wide range of neuroprotection. Our study aims to evaluate the effect of N-acetyl-L-tryptophan (NAT) in ameliorating the cognitive decline and neuroinflammation induced by Aβ 1-42 oligomers and to determine the therapeutic concentration of NAT in the brain. We administered Aβ 1-42 oligomers in rats via intracerebroventricular (i.c.v.) injection to induce AD-like conditions. The NAT-treated animals lowered the cognitive decline in the Morris water maze characterized by shorter escape latency and increased path efficiency and platform entries. Interestingly, the hippocampus and frontal cortex showed downregulation of tumor necrosis factor, interleukin-6, and substance P levels. NAT treatment also reduced acetylcholinesterase activity and total and phosphorylated nuclear factor kappa B and Tau levels. Lastly, we observed upregulation of cAMP response element-binding protein 1 (CREB1) signaling. Surprisingly, our HPLC method was not sensitive enough to detect the therapeutic levels of NAT in the brain, possibly due to NAT concentrations being below the lowest limit of quantification of our validated method. To summarize, the administration of NAT significantly lowered cognitive decline, neuroinflammatory pathways, and Tau protein and triggered the upregulation of CREB1 signaling, suggesting its neuroprotective role in AD-like conditions.

Amino-terminally elongated Aβ peptides are generated by the secreted metalloprotease ADAMTS4 and deposit in a subset of Alzheimer's disease brains

AIMS

The aggregation and deposition of amyloid-β (Aβ) peptides in the brain is thought to be the initial driver in the pathogenesis of Alzheimer's disease (AD). Aside from full-length Aβ peptides starting with an aspartate residue in position 1, both N-terminally truncated and elongated Aβ peptides are produced by various proteases from the amyloid precursor protein (APP) and have been detected in brain tissues and body fluids. Recently, we demonstrated that the particularly abundant N-terminally truncated Aβ4-x peptides are generated by ADAMTS4, a secreted metalloprotease that is exclusively expressed in the oligodendrocyte cell population. In this study, we investigated whether ADAMTS4 might also be involved in the generation of N-terminally elongated Aβ peptides.

METHODS

We used cell-free and cell-based assays in combination with matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF) and electrochemiluminescence sandwich immunoassays to identify and quantify N-terminally elongated Aβ peptide variants. Antibodies against these Aβ variants were characterised by peptide microarrays and employed for the immunohistochemical analyses of human brain samples.

RESULTS

In this study, we discovered additional ADAMTS4 cleavage sites in APP. These were located N-terminal to Asp-(1) in the Aβ peptide sequence between residues Glu-(-7) and Ile-(-6) as well as Glu-(-4) and Val-(-3), resulting in the release of N-terminally elongated Aβ-6-x and Aβ-3-x peptides, of which the latter serve as a component in a promising Aβ-based plasma biomarker. Aβ-6/-3-40 peptides were detected in supernatants of various cell lines and in the cerebrospinal fluid (CSF), and ADAMTS4 enzyme activity promoted the release of Aβ-6/-3-x peptides. Furthermore, by immunohistochemistry, a subset of AD cases displayed evidence of extracellular and vascular localization of N-terminally elongated Aβ-6/-3-x peptides.

DISCUSSION

The current findings implicate ADAMTS4 in both the pathological process of Aβ peptide aggregation and in the early detection of amyloid pathology in AD.

Identification of binding partners that facilitate membrane-type 5 matrix metalloproteinase (MT5-MMP) processing of amyloid precursor protein

One of the pathological hallmarks of Alzheimer's disease (AD) is the presence of extracellular deposits of amyloid beta (Aβ) peptide. In addition to Aβ as the core component of the amyloid plaque, the amyloid precursor protein (APP) processing fragment Aβ was also found accumulated around the plaque. The APPη pathway, mainly mediated by membrane-type 5 matrix metalloproteinase (MT5-MMP), represents an important factor in AD pathogenesis. The proamyloidogenic features of MT5-MMP could result from interactions with APP when trafficking between organelles, so determination of the location within the cell of APPη cleavage and interacting proteins of MT5-MMP affecting this process will be of priority in understanding the role of MT5-MMP in AD. In the present study, MT5-MMP was found to be located in the nucleus, cytosol, and cytosolic subcellular granules of CHO cells that stably expressed wild-type human APP751. MT5-MMP fusion proteins were constructed that could localize enzyme production in the Golgi apparatus, endosome, ER, mitochondria, or plasma membrane. The fusion proteins significantly increased sAPPη when directed to the endosome, Golgi apparatus, plasma membrane, or mitochondria. Since the C-terminal region of MT5-MMP is responsible for its intracellular location and trafficking, this domain was used as the bait in a yeast two-hybrid screen to identify MT5-MMP protein partners in a human brain cDNA library. Identified binding partners included N4BP2L1, TMX3, EIG121, bridging Integrator 1 (BIN1), RUFY4, HTRA1, and TMEM199. The binding of N4BP2L1, EIG121, BIN1, or TMX3 to MT5-MMP resulted in the most significant increase in sAPPη production. Thus, the action of MT5-MMP on APP occurs in multiple locations within the cell and is facilitated by site-specific binding partners.

The role of ATP-binding cassette transporter G1 (ABCG1) in Alzheimer's disease: A review of the mechanisms

The maintenance of cholesterol homeostasis is essential for central nervous system function. Consequently, factors that affect cholesterol homeostasis are linked to neurological disorders and pathologies. Among them, ATP-binding cassette transporter G1 (ABCG1) plays a significant role in atherosclerosis. However, its role in Alzheimer's disease (AD) is unclear. There is inconsistent information regarding ABCG1's role in AD. It can increase or decrease amyloid β (Aβ) levels in animals' brains. Clinical studies show that ABCG1 is involved in AD patients' impairment of cholesterol efflux capacity (CEC) in the cerebrospinal fluid (CSF). Lower Aβ levels in the CSF are correlated with ABCG1-mediated CEC dysfunction. ABCG1 modulates α-, β-, and γ-secretase activities in the plasma membrane and may affect Aβ production in the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) cell compartment. Despite contradictory findings regarding ABCG1's role in AD, this review shows that ABCG1 has a role in Aβ generation via modulation of membrane secretases. It is, however, necessary to investigate the underlying mechanism(s). ABCG1 may also contribute to AD pathology through its role in apoptosis and oxidative stress. As a result, ABCG1 plays a role in AD and is a candidate for drug development.

PCSK6 exacerbates Alzheimer's disease pathogenesis by promoting MT5-MMP maturation

Proprotein convertase subtilisin/kexin type 6 (PCSK6) is a calcium-dependent serine proteinase that regulates the proteolytic activity of various precursor proteins and facilitates protein maturation. Dysregulation of PCSK6 expression or function has been implicated in several pathological processes including nervous system diseases. However, whether and how PCSK6 is involved in the pathogenesis of Alzheimer's disease (AD) remains unclear. In this study, we reported that the expression of PCSK6 was significantly increased in the brain tissues of postmortem AD patients and APP23/PS45 transgenic AD model mice, as well as N2AAPP cells. Genetic knockdown of PCSK6 reduced amyloidogenic processing of APP in N2AAPP cells by suppressing the activation of membrane-type 5-matrix metalloproteinase (MT5-MMP), referred to as η-secretase. We further found that PCSK6 cleaved and activated MT5-MMP by recognizing the RRRNKR sequence in its N-terminal propeptide domain in N2A cells. The mutation or knockout of this cleavage motif prevented PCSK6 from interacting with MT5-MMP and performing cleavage. Importantly, genetic knockdown of PCSK6 with adeno-associated virus (AAV) reduced Aβ production and ameliorated hippocampal long-term potentiation (LTP) and long-term spatial learning and memory in APP23/PS45 transgenic mice. Taken together, these results demonstrate that genetic knockdown of PCSK6 effectively alleviate AD-related pathology and cognitive impairments by inactivating MT5-MMP, highlighting its potential as a novel therapeutic target for AD treatment.

Elossaily G, Vellapandian C, Prajapati B. Investigating the Potential Impact of Air Pollution on Alzheimer's Disease and the Utility of Multidimensional Imaging for Early Detection

Pollution is ubiquitous, and much of it is anthropogenic in nature, which is a severe risk factor not only for respiratory infections or asthma sufferers but also for Alzheimer's disease, which has received a lot of attention recently. This Review aims to investigate the primary environmental risk factors and their profound impact on Alzheimer's disease. It underscores the pivotal role of multidimensional imaging in early disease identification and prevention. Conducting a comprehensive review, we delved into a plethora of literature sources available through esteemed databases, including Science Direct, Google Scholar, Scopus, Cochrane, and PubMed. Our search strategy incorporated keywords such as "Alzheimer Disease", "Alzheimer's", "Dementia", "Oxidative Stress", and "Phytotherapy" in conjunction with "Criteria Pollutants", "Imaging", "Pathology", and "Particulate Matter". Alzheimer's disease is not only a result of complex biological factors but is exacerbated by the infiltration of airborne particles and gases that surreptitiously breach the nasal defenses to traverse the brain, akin to a Trojan horse. Various imaging modalities and noninvasive techniques have been harnessed to identify disease progression in its incipient stages. However, each imaging approach possesses inherent limitations, prompting exploration of a unified technique under a single umbrella. Multidimensional imaging stands as the linchpin for detecting and forestalling the relentless march of Alzheimer's disease. Given the intricate etiology of the condition, identifying a prospective candidate for Alzheimer's disease may take decades, rendering the development of a multimodal imaging technique an imperative. This research underscores the pressing need to recognize the chronic ramifications of invisible particulate matter and to advance our understanding of the insidious environmental factors that contribute to Alzheimer's disease.

Loss of the APP regulator RHBDL4 preserves memory in an Alzheimer's disease mouse model

Characteristic cerebral pathological changes of Alzheimer's disease (AD) such as glucose hypometabolism or the accumulation of cleavage products of the amyloid precursor protein (APP), known as Aβ peptides, lead to sustained endoplasmic reticulum (ER) stress and neurodegeneration. To preserve ER homeostasis, cells activate their unfolded protein response (UPR). The rhomboid-like-protease 4 (RHBDL4) is an enzyme that participates in the UPR by targeting proteins for proteasomal degradation. We demonstrated previously that RHBLD4 cleaves APP in HEK293T cells, leading to decreased total APP and Aβ. More recently, we showed that RHBDL4 processes APP in mouse primary mixed cortical cultures as well. Here, we aim to examine the physiological relevance of RHBDL4 in the brain. We first found that brain samples from AD patients and an AD mouse model (APPtg) showed increased RHBDL4 mRNA and protein expression. To determine the effects of RHBDL4's absence on APP physiology in vivo, we crossed APPtg mice to a RHBDL4 knockout (R4 KO) model. RHBDL4 deficiency in APPtg mice led to increased total cerebral APP and Aβ levels when compared to APPtg controls. Contrary to expectations, as assessed by cognitive tests, RHBDL4 absence rescued cognition in 5-month-old female APPtg mice. Informed by unbiased RNAseq data, we demonstrated in vitro and in vivo that RHBDL4 absence leads to greater levels of active β-catenin due to decreased proteasomal clearance. Decreased β-catenin activity is known to underlie cognitive defects in APPtg mice and AD. Our work suggests that RHBDL4's increased expression in AD, in addition to regulating APP levels, leads to aberrant degradation of β-catenin, contributing to cognitive impairment.

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.

miR-429-3p mediates memory decline by targeting MKP-1 to reduce surface GluA1-containing AMPA receptors in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a leading cause of dementia in the elderly. Mitogen-activated protein kinase phosphatase 1 (MKP-1) plays a neuroprotective role in AD. However, the molecular mechanisms underlying the effects of MKP-1 on AD have not been extensively studied. MicroRNAs (miRNAs) regulate gene expression at the post-transcriptional level, thereby repressing mRNA translation. Here, we reported that the microRNA-429-3p (miR-429-3p) was significantly increased in the brain of APP23/PS45 AD model mice and N2AAPP AD model cells. We further found that miR-429-3p could downregulate MKP-1 expression by directly binding to its 3'-untranslated region (3' UTR). Inhibition of miR-429-3p by its antagomir (A-miR-429) restored the expression of MKP-1 to a control level and consequently reduced the amyloidogenic processing of APP and Aβ accumulation. More importantly, intranasal administration of A-miR-429 successfully ameliorated the deficits of hippocampal CA1 long-term potentiation and spatial learning and memory in AD model mice by suppressing extracellular signal-regulated kinase (ERK1/2)-mediated GluA1 hyperphosphorylation at Ser831 site, thereby increasing the surface expression of GluA1-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). Together, these results demonstrate that inhibiting miR-429-3p to upregulate MKP-1 effectively improves cognitive and synaptic functions in AD model mice, suggesting that miR-429/MKP-1 pathway may be a novel therapeutic target for AD treatment.

Re-Arranging the Puzzle between the Amyloid-Beta and Tau Pathology: An APP-Centric Approach

After several years of research in the field of Alzheimer's disease (AD), it is still unclear how amyloid-beta (Aβ) and Tau, two key hallmarks of the disease, mediate the neuropathogenic events that lead to AD. Current data challenge the "Amyloid Cascade Hypothesis" that has prevailed in the field of AD, stating that Aβ precedes and triggers Tau pathology that will eventually become the toxic entity in the progression of the disease. This perspective also led the field of therapeutic approaches towards the development of strategies that target Aβ or Tau. In the present review, we discuss recent literature regarding the neurotoxic role of both Aβ and Tau in AD, as well as their physiological function in the healthy brain. Consequently, we present studies suggesting that Aβ and Tau act independently of each other in mediating neurotoxicity in AD, thereafter, re-evaluating the "Amyloid Cascade Hypothesis" that places Tau pathology downstream of Aβ. More recent studies have confirmed that both Aβ and Tau could propagate the disease and induce synaptic and memory impairments via the amyloid precursor protein (APP). This finding is not only interesting from a mechanistic point of view since it provides better insights into the AD pathogenesis but also from a therapeutic point of view since it renders APP a common downstream effector for both Aβ and Tau. Subsequently, therapeutic strategies that act on APP might provide a more viable and physiologically relevant approach for targeting AD.

Iron and Targeted Iron Therapy in Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disease worldwide. β-amyloid plaque (Aβ) deposition and hyperphosphorylated tau, as well as dysregulated energy metabolism in the brain, are key factors in the progression of AD. Many studies have observed abnormal iron accumulation in different regions of the AD brain, which is closely correlated with the clinical symptoms of AD; therefore, understanding the role of brain iron accumulation in the major pathological aspects of AD is critical for its treatment. This review discusses the main mechanisms and recent advances in the involvement of iron in the above pathological processes, including in iron-induced oxidative stress-dependent and non-dependent directions, summarizes the hypothesis that the iron-induced dysregulation of energy metabolism may be an initiating factor for AD, based on the available evidence, and further discusses the therapeutic perspectives of targeting iron.

Amyloid-Beta Peptides Trigger Premature Functional and Gene Expression Alterations in Human-Induced Neurons

Alzheimer's disease (AD) is the most prevalent cause of dementia in the elderly, characterized by the presence of amyloid-beta (Aβ) plaques, neurofibrillary tangles, neuroinflammation, synapse loss and neurodegeneration in the brain. The amyloid cascade hypothesis postulates that deposition of Aβ peptides is the causative agent of AD pathology, but we still lack comprehensive understanding of the molecular mechanisms connecting Aβ peptides to neuronal dysfunctions in AD. In this work, we investigate the early effects of Aβ peptide accumulation on the functional properties and gene expression profiles of human-induced neurons (hiNs). We show that hiNs acutely exposed to low concentrations of both cell-secreted Aβ peptides or synthetic Aβ1-42 exhibit alterations in the frequency of calcium transients suggestive of increased neuronal excitability. Using single-cell RNA sequencing, we also show that cell-secreted Aβ up-regulates the expression of several synapse-related genes and down-regulates the expression of genes associated with metabolic stress mainly in glutamatergic neurons and, to a lesser degree, in GABAergic neurons and astrocytes. These neuronal alterations correlate with activation of the SEMA5, EPHA and NECTIN signaling pathways, which are important regulators of synaptic plasticity. Altogether, our findings indicate that slight elevations in Aβ concentrations are sufficient to elicit transcriptional changes in human neurons, which can contribute to early alterations in neural network activity.

Evidence for a clathrin-independent endocytic pathway for APP internalization in the neuronal somatodendritic compartment

Amyloid precursor protein (APP) internalization via clathrin-/dynamin-mediated endocytosis (CME) mediated by its YENPTY motif into endosomes containing β-secretase is proposed to be critical for amyloid-beta (Aβ) production. Here, we show that somatodendritic APP internalization in primary rodent neurons is not blocked by inhibiting dynamin or mutating the YENPTY motif, in contrast to non-neuronal cell lines. These phenomena, confirmed in induced human neurons under dynamin inhibition, occur during basal conditions and chemical long-term-depression stimulus, pointing to a clathrin-independent internalization pathway for somatodendritic APP. Mutating the YENPTY motif does not alter APP recycling, degradation, or endolysosomal colocalization. However, both dynamin inhibition and the YENPTY mutant significantly decrease secreted Aβ in neurons, suggesting that internalized somatodendritic APP may not constitute a major source of Aβ. Interestingly, like APP, somatodendritic low-density lipoprotein receptor (LDLR) internalization does not require its CME motif. These results highlight intriguing differences in neuronal internalization pathways and refine our understanding of Aβ production and secretion.

C/EBPβ/AEP Signaling Drives Alzheimer's Disease Pathogenesis

Alzheimer's disease (AD) is the most common type of dementia. Almost two-thirds of patients with AD are female. The reason for the higher susceptibility to AD onset in women is unclear. However, hormone changes during the menopausal transition are known to be associated with AD. Most recently, we reported that follicle-stimulating hormone (FSH) promotes AD pathology and enhances cognitive dysfunctions via activating the CCAAT-enhancer-binding protein (C/EBPβ)/asparagine endopeptidase (AEP) pathway. This review summarizes our current understanding of the crucial role of the C/EBPβ/AEP pathway in driving AD pathogenesis by cleaving multiple critical AD players, including APP and Tau, explaining the roles and the mechanisms of FSH in increasing the susceptibility to AD in postmenopausal females. The FSH-C/EBPβ/AEP pathway may serve as a novel therapeutic target for the treatment of AD.

Strategic outline of interventions targeting extracellular matrix for promoting healthy longevity

The extracellular matrix (ECM), composed of interlinked proteins outside of cells, is an important component of the human body that helps maintain tissue architecture and cellular homeostasis. As people age, the ECM undergoes changes that can lead to age-related morbidity and mortality. Despite its importance, ECM aging remains understudied in the field of geroscience. In this review, we discuss the core concepts of ECM integrity, outline the age-related challenges and subsequent pathologies and diseases, summarize diagnostic methods detecting a faulty ECM, and provide strategies targeting ECM homeostasis. To conceptualize this, we built a technology research tree to hierarchically visualize possible research sequences for studying ECM aging. This strategic framework will hopefully facilitate the development of future research on interventions to restore ECM integrity, which could potentially lead to the development of new drugs or therapeutic interventions promoting health during aging.

The Labyrinthine Landscape of APP Processing: State of the Art and Possible Novel Soluble APP-Related Molecular Players in Traumatic Brain Injury and Neurodegeneration

Amyloid Precursor Protein (APP) and its cleavage processes have been widely investigated in the past, in particular in the context of Alzheimer’s Disease (AD). Evidence of an increased expression of APP and its amyloidogenic-related cleavage enzymes, β-secretase 1 (BACE1) and γ-secretase, at the hit axon terminals following Traumatic Brain Injury (TBI), firstly suggested a correlation between TBI and AD. Indeed, mild and severe TBI have been recognised as influential risk factors for different neurodegenerative diseases, including AD. In the present work, we describe the state of the art of APP proteolytic processing, underlining the different roles of its cleavage fragments in both physiological and pathological contexts. Considering the neuroprotective role of the soluble APP alpha (sAPPα) fragment, we hypothesised that sAPPα could modulate the expression of genes of interest for AD and TBI. Hence, we present preliminary experiments addressing sAPPα-mediated regulation of BACE1, Isthmin 2 (ISM2), Tetraspanin-3 (TSPAN3) and the Vascular Endothelial Growth Factor (VEGFA), each discussed from a biological and pharmacological point of view in AD and TBI. We finally propose a neuroprotective interaction network, in which the Receptor for Activated C Kinase 1 (RACK1) and the signalling cascade of PKCβII/nELAV/VEGF play hub roles, suggesting that vasculogenic-targeting therapies could be a feasible approach for vascular-related brain injuries typical of AD and TBI.

The η-secretase-derived APP fragment ηCTF is localized in Golgi, endosomes and extracellular vesicles and contributes to Aβ production

The processing of the amyloid precursor protein (APP) is one of the key events contributing to Alzheimer's disease (AD) etiology. Canonical cleavages by β- and γ-secretases lead to Aβ production which accumulate in amyloid plaques. Recently, the matrix metalloprotease MT5-MMP, referred to as η-secretase, has been identified as a novel APP cleaving enzyme producing a transmembrane fragment, ηCTF that undergoes subsequent cleavages by α- and β-secretases yielding the Aηα and Aηβ peptides, respectively. The functions and contributions of ηCTF and its related fragments to AD pathology are poorly understood. In this study, we designed a novel immunological probe referred to as ηCTF-NTer antibody that specifically interacts with the N-terminal part of ηCTF targeting ηCTF, Aηα, Aηβ but not C99, C83 and Aβ. We examined the fate and localization of ηCTF fragment in various cell models and in mice. We found that overexpressed ηCTF undergoes degradation in the proteasomal and autophagic pathways and accumulates mainly in the Golgi and in endosomes. Moreover, we observed the presence of ηCTF in small extracellular vesicles purified from neuroblastoma cells or from mouse brains expressing ηCTF. Importantly, the expression of ηCTF in fibroblasts devoid on APP leads to Aβ production demonstrating its contribution to the amyloidogenic pathway. Finally, we observed an ηCTF-like immunoreactivity around amyloid plaques and an age-dependent accumulation of ηCTF in the triple-transgenic mouse AD model. Thus, our study suggests that the ηCTF fragment likely contributes to AD pathology by its exosomal spreading and involvement in Aβ production.

Age-dependent NMDA receptor function is regulated by the amyloid precursor protein

N-methyl-D-aspartate receptors (NMDARs) are critical for the maturation and plasticity of glutamatergic synapses. In the hippocampus, NMDARs mainly contain GluN2A and/or GluN2B regulatory subunits. The amyloid precursor protein (APP) has emerged as a putative regulator of NMDARs, but the impact of this interaction to their function is largely unknown. By combining patch-clamp electrophysiology and molecular approaches, we unravel a dual mechanism by which APP controls GluN2B-NMDARs, depending on the life stage. We show that APP is highly abundant specifically at the postnatal postsynapse. It interacts with GluN2B-NMDARs, controlling its synaptic content and mediated currents, both in infant mice and primary neuronal cultures. Upon aging, the APP amyloidogenic-derived C-terminal fragments, rather than APP full-length, contribute to aberrant GluN2B-NMDAR currents. Accordingly, we found that the APP processing is increased upon aging, both in mice and human brain. Interfering with stability or production of the APP intracellular domain normalized the GluN2B-NMDARs currents. While the first mechanism might be essential for synaptic maturation during development, the latter could contribute to age-related synaptic impairments.

Ex vivo analysis platforms for monitoring amyloid precursor protein cleavage

Alzheimer's disease (AD) is a progressive neurodegenerative brain disorder and the most common cause of dementia in the elderly. The presence of large numbers of senile plaques, neurofibrillary tangles, and cerebral atrophy is the characteristic feature of AD. Amyloid β peptide (Aβ), derived from the amyloid precursor protein (APP), is the main component of senile plaques. AD has been extensively studied using methods involving cell lines, primary cultures of neural cells, and animal models; however, discrepancies have been observed between these methods. Dissociated cultures lose the brain's tissue architecture, including neural circuits, glial cells, and extracellular matrix. Experiments with animal models are lengthy and require laborious monitoring of multiple parameters. Therefore, it is necessary to combine these experimental models to understand the pathology of AD. An experimental platform amenable to continuous observation and experimental manipulation is required to analyze long-term neuronal development, plasticity, and progressive neurodegenerative diseases. In the current study, we provide a practical method to slice and cultivate rodent hippocampus to investigate the cleavage of APP and secretion of Aβ in an ex vivo model. Furthermore, we provide basic information on Aβ secretion using slice cultures. Using our optimized method, dozens to hundreds of long-term stable slice cultures can be coordinated simultaneously. Our findings are valuable for analyses of AD mouse models and senile plaque formation culture models.

Cerebral amyloid angiopathy-related cardiac injury: Focus on cardiac cell death

Cerebral amyloid angiopathy (CAA) is a kind of disease in which amyloid β (Aβ) and other amyloid protein deposits in the cerebral cortex and the small blood vessels of the brain, causing cerebrovascular and brain parenchymal damage. CAA patients are often accompanied by cardiac injury, involving Aβ, tau and transthyroxine amyloid (ATTR). Aβ is the main injury factor of CAA, which can accelerate the formation of coronary artery atherosclerosis, aortic valve osteogenesis calcification and cardiomyocytes basophilic degeneration. In the early stage of CAA (pre-stroke), the accompanying locus coeruleus (LC) amyloidosis, vasculitis and circulating Aβ will induce first hit to the heart. When the CAA progresses to an advanced stage and causes a cerebral hemorrhage, the hemorrhage leads to autonomic nervous function disturbance, catecholamine surges, and systemic inflammation reaction, which can deal the second hit to the heart. Based on the brain-heart axis, CAA and its associated cardiac injury can create a vicious cycle that accelerates the progression of each other.

AP2S1 regulates APP degradation through late endosome-lysosome fusion in cells and APP/PS1 mice

AP2S1 is the sigma 2 subunit of adaptor protein 2 (AP2) that is essential for endocytosis. In this study, we investigated the potential role of AP2S1 in intracellular processing of amyloid precursor protein (APP), which contributes to the pathogenesis of Alzheimer disease (AD) by generating the toxic β-amyloid peptide (Aβ). We found that knockdown or overexpression of AP2S1 decreased or increased the protein levels of APP and Aβ in cells stably expressing human full-length APP695, respectively. This effect was unrelated to endocytosis but involved lysosomal degradation. Morphological studies revealed that silencing of AP2S1 promoted the translocalization of APP from RAB9-positive late endosomes (LE) to LAMP1-positive lysosomes, which was paralleled by the enhanced LE-lysosome fusion. In support, silencing of vacuolar protein sorting-associated protein 41 (VPS41) that is implicated in LE-lyso fusion prevented AP2S1-mediated regulation of APP degradation and translocalization. In APP/PS1 mice, an animal model of AD, AAV-mediated delivery of AP2S1 shRNA in the hippocampus significantly reduced the protein levels of APP and Aβ, with the concomitant APP translocalization, LE-lyso fusion and the improved cognitive functions. Taken together, these data uncover a LE-lyso fusion mechanism in APP degradation and suggest a novel role for AP2S1 in the pathophysiology of AD.

Research Progress on the Etiology and Pathogenesis of Alzheimer's Disease from the Perspective of Chronic Stress

Due to its extremely complex pathogenesis, no effective drugs to prevent, delay progression, or cure Alzheimer's disease (AD) exist at present. The main pathological features of AD are senile plaques composed of β-amyloid, neurofibrillary tangles formed by hyperphosphorylation of the tau protein, and degeneration or loss of neurons in the brain. Many risk factors associated with the onset of AD, including gene mutations, aging, traumatic brain injury, endocrine and cardiovascular diseases, education level, and obesity. Growing evidence points to chronic stress as one of the major risk factors for AD, as it can promote the onset and development of AD-related pathologies via a mechanism that is not well known. The use of murine stress models, including restraint, social isolation, noise, and unpredictable stress, has contributed to improving our understanding of the relationship between chronic stress and AD. This review summarizes the evidence derived from murine models on the pathological features associated with AD and the related molecular mechanisms induced by chronic stress. These results not only provide a retrospective interpretation for understanding the pathogenesis of AD, but also provide a window of opportunity for more effective preventive and identifying therapeutic strategies for stress-induced AD.

Therapeutic approaches of nutraceuticals in the prevention of Alzheimer's disease

Alzheimer's disease (AD) is a neurological illness that causes memory loss over time. Currently, available pharmaceutical medicines and products are limited, and they have side effects at a higher price. Researchers and scientists have observed significant effects of nutraceuticals. Various preclinical and clinical studies were investigated for the Anti-Alzheimer's activity of nutraceuticals. The increasing ability of the pathogenesis of AD has led to the analysis of novel therapeutic targets, including the pathophysiological mechanisms and distinct cascades. So, current improvement will show the most adequate and prominent nutraceuticals and suggested concise mechanisms involving autophagy regulation, anti-inflammatory, antioxidant, mitochondrial homeostasis, and others. The effects of nutraceuticals cannot be ignored; it is important to investigate high-quality clinical trials. Given the potential of nutraceuticals to battle AD as multi-targeted therapies, it's vital to evaluate them as viable lead compounds for drug discovery and development. To the best of the authors 'knowledge, modification of blood-brain barrier permeability, bioavailability, and aspects of randomized clinical trials should be considered in prospective investigations. PRACTICAL APPLICATIONS: Advancements in molecular diagnostic and fundamentals have implemented particular usefulness for drug evaluation. An excess of experimental knowledge occurs regarding the effect of nutraceuticals on AD. There are various preclinical and clinical studies that have been done on nutraceuticals. In addition, various substitute inhibit and enhance some pathophysiological levels associated with AD. Nutraceuticals are easily available and have fewer side effects with cost-effective advantages. However, further investigations and clinical trials are required to encourage its effect on disease.

Synthetic, Cell-Derived, Brain-Derived, and Recombinant β-Amyloid: Modelling Alzheimer's Disease for Research and Drug Development

Alzheimer's disease (AD) is the most common cause of dementia in the elderly, characterised by the accumulation of senile plaques and tau tangles, neurodegeneration, and neuroinflammation in the brain. The development of AD is a pathological cascade starting according to the amyloid hypothesis with the accumulation and aggregation of the β-amyloid peptide (Aβ), which induces hyperphosphorylation of tau and promotes the pro-inflammatory activation of microglia leading to synaptic loss and, ultimately, neuronal death. Modelling AD-related processes is important for both studying the molecular basis of the disease and the development of novel therapeutics. The replication of these processes is often achieved with the use of a purified Aβ peptide. However, Aβ preparations obtained from different sources can have strikingly different properties. This review aims to compare the structure and biological effects of Aβ oligomers and aggregates of a higher order: synthetic, recombinant, purified from cell culture, or extracted from brain tissue. The authors summarise the applicability of Aβ preparations for modelling Aβ aggregation, neurotoxicity, cytoskeleton damage, receptor toxicity in vitro and cerebral amyloidosis, synaptic plasticity disruption, and cognitive impairment in vivo and ex vivo. Further, the paper discusses the causes of the reported differences in the effect of Aβ obtained from the sources mentioned above. This review points to the importance of the source of Aβ for AD modelling and could help researchers to choose the optimal way to model the Aβ-induced abnormalities.

Amyloidogenesis: What Do We Know So Far?

The study of protein aggregation, and amyloidosis in particular, has gained considerable interest in recent times. Several neurodegenerative diseases, such as Alzheimer's (AD) and Parkinson's (PD) show a characteristic buildup of proteinaceous aggregates in several organs, especially the brain. Despite the enormous upsurge in research articles in this arena, it would not be incorrect to say that we still lack a crystal-clear idea surrounding these notorious aggregates. In this review, we attempt to present a holistic picture on protein aggregation and amyloids in particular. Using a chronological order of discoveries, we present the case of amyloids right from the onset of their discovery, various biophysical techniques, including analysis of the structure, the mechanisms and kinetics of the formation of amyloids. We have discussed important questions on whether aggregation and amyloidosis are restricted to a subset of specific proteins or more broadly influenced by the biophysiochemical and cellular environment. The therapeutic strategies and the significant failure rate of drugs in clinical trials pertaining to these neurodegenerative diseases have been also discussed at length. At a time when the COVID-19 pandemic has hit the globe hard, the review also discusses the plausibility of the far-reaching consequences posed by the virus, such as triggering early onset of amyloidosis. Finally, the application(s) of amyloids as useful biomaterials has also been discussed briefly in this review.

Advancement in Analytical Techniques for Determining the Activity of β-Site Amyloid Precursor Protein Cleaving Enzyme 1

Alzheimer's disease (AD) is a degenerative disease of the central nervous system. The pathogenesis is still not fully clear. One of the main histopathological manifestations is senile plaques formed by β-amyloid (Aβ) accumulation. Aβ is generated from the sequential proteolysis of amyloid precursor protein (APP) by β-secretase [i.e. β-site APP cleaving enzyme 1 (BACE1)] and γ-secretase, with a rate-limiting step controlled by BACE1 activity. Therefore, inhibiting BACE1 activity has become a potential therapeutic strategy for AD. The development of reliable detection methods for BACE1 activity plays an important role in early diagnosis of AD and evaluation of the therapeutic effect of new drugs for AD. This article has reviewed the recent advances in BACE1 activity detection techniques. The challenges of applying these analysis techniques to early clinical diagnosis of AD and development trends of the detection techniques have been prospected.

Pathological and neurophysiological outcomes of seeding human-derived tau pathology in the APP-KI NL-G-F and NL-NL mouse models of Alzheimer's Disease

The two main histopathological hallmarks that characterize Alzheimer’s Disease are the presence of amyloid plaques and neurofibrillary tangles. One of the current approaches to studying the consequences of amyloid pathology relies on the usage of transgenic animal models that incorporate the mutant humanized form of the amyloid precursor protein (hAPP), with animal models progressively developing amyloid pathology as they age. However, these mice models generally overexpress the hAPP protein to facilitate the development of amyloid pathology, which has been suggested to elicit pathological and neuropathological changes unrelated to amyloid pathology. In this current study, we characterized APP knock-in (APP-KI) animals, that do not overexpress hAPP but still develop amyloid pathology to understand the influence of protein overexpression. We also induced tau pathology via human-derived tau seeding material to understand the neurophysiological effects of amyloid and tau pathology. We report that tau-seeded APP-KI animals progressively develop tau pathology, exacerbated by the presence of amyloid pathology. Interestingly, older amyloid-bearing, tau-seeded animals exhibited more amyloid pathology in the entorhinal area, isocortex and hippocampus, but not thalamus, which appeared to correlate with impairments in gamma oscillations before seeding. Tau-seeded animals also featured immediate deficits in power spectra values and phase-amplitude indices in the hippocampus after seeding, with gamma power spectra deficits persisting in younger animals. Both deficits in hippocampal phase-amplitude coupling and gamma power differentiate tau-seeded, amyloid-positive animals from buffer controls. Based on our results, impairments in gamma oscillations appear to be strongly associated with the presence and development of amyloid and tau pathology, and may also be an indicator of neuropathology, network dysfunction, and even potential disposition to the future development of amyloid pathology.

Quantitative Proteomics Reveals That ADAM15 Can Have Proteolytic-Independent Functions in the Steady State

A disintegrin and metalloproteinase 15 (ADAM15) is a member of the ADAM family of sheddases. Its genetic ablation in mice suggests that ADAM15 plays an important role in a wide variety of biological functions, including cartilage homeostasis. Nevertheless, while the substrate repertoire of other members of the ADAM family, including ADAM10 and ADAM17, is largely established, little is known about the substrates of ADAM15 and how it exerts its biological functions. Herein, we used unbiased proteomics to identify ADAM15 substrates and proteins regulated by the proteinase in chondrocyte-like HTB94 cells. ADAM15 silencing did not induce major changes in the secretome composition of HTB94 cells, as revealed by two different proteomic approaches. Conversely, overexpression of ADAM15 remodeled the secretome, with levels of several secreted proteins being altered compared to GFP-overexpressing controls. However, the analysis did not identify potential substrates of the sheddase, i.e., transmembrane proteins released by ADAM15 in the extracellular milieu. Intriguingly, secretome analysis and immunoblotting demonstrated that ADAM15 overexpression increased secreted levels of tissue inhibitor of metalloproteinases 3 (TIMP-3), a major regulator of extracellular matrix turnover. An inactive form of ADAM15 led to a similar increase in the inhibitor, indicating that ADAM15 regulates TIMP-3 secretion by an unknown mechanism independent of its catalytic activity. In conclusion, high-resolution quantitative proteomics of HTB94 cells manipulated to have increased or decreased ADAM15 expression did not identify canonical substrates of the proteinase in the steady state, but it revealed that ADAM15 can modulate the secretome in a catalytically-independent manner.

Autoantibodies targeting neuronal proteins as biomarkers for neurodegenerative diseases

Neurodegenerative diseases (NDDs) are associated with the accumulation of a range of misfolded proteins across the central nervous system and related autoimmune responses, including the generation of antibodies and the activation of immune cells. Both innate and adaptive immunity become mobilized, leading to cellular and humoral effects. The role of humoral immunity in disease onset and progression remains to be elucidated with rising evidence suggestive of positive (protection, repair) and negative (injury, toxicity) outcomes. In this study, we review advances in research of neuron-targeting autoantibodies in the most prevalent NDDs. We discuss their biological origin, molecular diversity and changes in the course of diseases, consider their relevance to the initiation and progression of pathology as well as diagnostic and prognostic significance. It is suggested that the emerging autoimmune aspects of NDDs not only could facilitate the early detection but also might help to elucidate previously unknown facets of pathobiology with relevance to the development of precision medicine.

The amyloid precursor protein: a converging point in Alzheimer's disease

The decades of evidence that showcase the role of amyloid precursor protein (APP), and its fragment amyloidβ (Aβ), in Alzheimer's disease (AD) pathogenesis are irrefutable. However, the absolute focus on the single APP metabolite Aβ as the cause for AD has resulted in APP and its other fragments that possess toxic propensity, to be overlooked as targets for treatment. The complexity of its processing and its association with systematic metabolism suggests that, if misregulated, APP has the potential to provoke an array of metabolic dysfunctions. This review discusses APP and several of its cleaved products with a particular focus on their toxicity and ability to disrupt healthy cellular function, in relation to AD development. We subsequently argue that the reduction of APP, which would result in a concurrent decrease in Aβ as well as all other toxic APP metabolites, would alleviate the toxic environment associated with AD and slow disease progression. A discussion of those drug-like compounds already identified to possess this capacity is also included.

Natural Bioactive Products and Alzheimer’s Disease Pathology: Lessons from Caenorhabditis elegans Transgenic Models

Alzheimer’s disease (AD) is an age-dependent, progressive disorder affecting millions of people. Currently, the therapeutics for AD only treat the symptoms. Although they have been used to discover new products of interest for this disease, mammalian models used to investigate the molecular determinants of this disease are often prohibitively expensive, time-consuming and very complex. On the other hand, cell cultures lack the organism complexity involved in AD. Given the highly conserved neurological pathways between mammals and invertebrates, Caenorhabditis elegans has emerged as a powerful tool for the investigation of the pathophysiology of human AD. Numerous models of both Tau- and Aβ-induced toxicity, the two prime components observed to correlate with AD pathology and the ease of performing RNA interference for any gene in the C. elegans genome, allow for the identification of multiple therapeutic targets. The effects of many natural products in main AD hallmarks using these models suggest promising health-promoting effects. However, the way in which they exert such effects is not entirely clear. One of the reasons is that various possible therapeutic targets have not been evaluated in many studies. The present review aims to explore shared therapeutical targets and the potential of each of them for AD treatment or prevention.

Noise as a cause of neurodegenerative disorders: molecular and cellular mechanisms

Noise as an environmental stressor becomes of increasing importance in our industrialized world, and especially traffic noise from the environment represents a potential novel neurodegenerative risk factor, as well as for hearing loss. A significant number of studies have been suggested that the overproduction of reactive oxygen species (ROS) has a complex role in stimulation of pathologic events. Experimental studies upon molecular pathways of traffic noise exposure proposed that it increased the level of stress hormones and mediated the inflammatory and oxidative stress (OS) pathways resulting in endothelial and neuronal dysfunction. Studies have shown that neurons are especially sensitive to OS due to high polyunsaturated fatty acids content in membranes, high oxygen uptake, and weak antioxidant defense. However, OS induces the necrotic and apoptotic cell deaths in the cochlea. Chronic noise is one of the many overall reasons of obtained sensorineural hearing loss which destroys cognitive functions in human and animals, as well as suppresses neurogenesis in the hippocampus. Nevertheless, behavioral disorders caused by noise are mainly accompanied with oxidative stress, but the clear molecular mechanism of neurodegeneration due to disruption of the pro- and antioxidant systems is still not fully understood. This paper aims to highlight the down-stream pathophysiology of noise-induced mental disorders, including hearing loss, annoyance, anxiety, depression, memory loss, and Alzheimer's disease, describing the underlying mechanisms of induction of inflammation and oxidative stress.

Parkin as a Molecular Bridge Linking Alzheimer’s and Parkinson’s Diseases?

Alzheimer’s (AD) and Parkinson’s (PD) diseases are two distinct age-related pathologies that are characterized by various common dysfunctions. They are referred to as proteinopathies characterized by ubiquitinated protein accumulation and aggregation. This accumulation is mainly due to altered lysosomal and proteasomal clearing processes and is generally accompanied by ER stress disturbance, autophagic and mitophagic defects, mitochondrial structure and function alterations and enhanced neuronal cell death. Genetic approaches aimed at identifying molecular triggers responsible for familial forms of AD or PD have helped to understand the etiology of their sporadic counterparts. It appears that several proteins thought to contribute to one of these pathologies are also likely to contribute to the other. One such protein is parkin (PK). Here, we will briefly describe anatomical lesions and genetic advances linked to AD and PD as well as the main cellular processes commonly affected in these pathologies. Further, we will focus on current studies suggesting that PK could well participate in AD and thereby act as a molecular bridge between these two pathologies. In particular, we will focus on the transcription factor function of PK and its newly described transcriptional targets that are directly related to AD- and PD-linked cellular defects.

Knockdown of Amyloid Precursor Protein: Biological Consequences and Clinical Opportunities

Amyloid precursor protein (APP) and its cleavage fragment Amyloid-β (Aβ) have fundamental roles in Alzheimer's disease (AD). Genetic alterations that either increase the overall dosage of APP or alter its processing to favour the generation of longer, more aggregation prone Aβ species, are directly causative of the disease. People living with one copy of APP are asymptomatic and reducing APP has been shown to lower the relative production of aggregation-prone Aβ species in vitro. For these reasons, reducing APP expression is an attractive approach for AD treatment and prevention. In this review, we will describe the structure and the known functions of APP and go on to discuss the biological consequences of APP knockdown and knockout in model systems. We highlight progress in therapeutic strategies to reverse AD pathology via reducing APP expression. We conclude that new technologies that reduce the dosage of APP expression may allow disease modification and slow clinical progression, delaying or even preventing onset.

MT5-MMP promotes neuroinflammation, neuronal excitability and Aβ production in primary neuron/astrocyte cultures from the 5xFAD mouse model of Alzheimer’s disease

Background

Membrane-type matrix metalloproteinase 5 (MT5-MMP) deficiency in the 5xFAD mouse model of Alzheimer's disease (AD) reduces brain neuroinflammation and amyloidosis, and prevents deficits in synaptic activity and cognition in prodromal stages of the disease. In addition, MT5-MMP deficiency prevents interleukin-1 beta (IL-1β)-mediated inflammation in the peripheral nervous system. In this context, we hypothesized that the MT5-MMP/IL-1β tandem could regulate nascent AD pathogenic events in developing neural cells shortly after the onset of transgene activation.

Methods

To test this hypothesis, we used 11–14 day in vitro primary cortical cultures from wild type, MT5-MMP−/−, 5xFAD and 5xFAD/MT5-MMP−/− mice, and evaluated the impact of MT5-MMP deficiency and IL-1β treatment for 24 h, by performing whole cell patch-clamp recordings, RT-qPCR, western blot, gel zymography, ELISA, immunocytochemistry and adeno-associated virus (AAV)-mediated transduction.

Results

5xFAD cells showed higher levels of MT5-MMP than wild type, concomitant with higher basal levels of inflammatory mediators. Moreover, MT5-MMP-deficient cultures had strong decrease of the inflammatory response to IL-1β, as well as decreased stability of recombinant IL-1β. The levels of amyloid beta peptide (Aβ) were similar in 5xFAD and wild-type cultures, and IL-1β treatment did not affect Aβ levels. Instead, the absence of MT5-MMP significantly reduced Aβ by more than 40% while sparing APP metabolism, suggesting altogether no functional crosstalk between IL-1β and APP/Aβ, as well as independent control of their levels by MT5-MMP. The lack of MT5-MMP strongly downregulated the AAV-induced neuronal accumulation of the C-terminal APP fragment, C99, and subsequently that of Aβ. Finally, MT5-MMP deficiency prevented basal hyperexcitability observed in 5xFAD neurons, but not hyperexcitability induced by IL-1β treatment.

Conclusions

Neuroinflammation and hyperexcitability precede Aβ accumulation in developing neural cells with nascent expression of AD transgenes. MT5-MMP deletion is able to tune down basal neuronal inflammation and hyperexcitability, as well as APP/Aβ metabolism. In addition, MT5-MMP deficiency prevents IL-1β-mediated effects in brain cells, except hyperexcitability. Overall, this work reinforces the idea that MT5-MMP is at the crossroads of pathogenic AD pathways that are already incipiently activated in developing neural cells, and that targeting MT5-MMP opens interesting therapeutic prospects.

Brothers in arms: proBDNF/BDNF and sAPPα/Aβ-signaling and their common interplay with ADAM10, TrkB, p75NTR, sortilin, and sorLA in the progression of Alzheimer's disease

Brain-derived neurotrophic factor (BDNF) is an important modulator for a variety of functions in the central nervous system (CNS). A wealth of evidence, such as reduced mRNA and protein level in the brain, cerebrospinal fluid (CSF), and blood samples of Alzheimer's disease (AD) patients implicates a crucial role of BDNF in the progression of this disease. Especially, processing and subcellular localization of BDNF and its receptors TrkB and p75 are critical determinants for survival and death in neuronal cells. Similarly, the amyloid precursor protein (APP), a key player in Alzheimer's disease, and its cleavage fragments sAPPα and Aβ are known for their respective roles in neuroprotection and neuronal death. Common features of APP- and BDNF-signaling indicate a causal relationship in their mode of action. However, the interconnections of APP- and BDNF-signaling are not well understood. Therefore, we here discuss dimerization properties, localization, processing by α- and γ-secretase, relevance of the common interaction partners TrkB, p75, sorLA, and sortilin as well as shared signaling pathways of BDNF and sAPPα.

Synaptic dysfunction in early phases of Alzheimer's Disease

The synapse is the locus of plasticity where short-term alterations in synaptic strength are converted to long-lasting memories. In addition to the presynaptic terminal and the postsynaptic compartment, a more holistic view of the synapse includes the astrocytes and the extracellular matrix to form a tetrapartite synapse. All these four elements contribute to synapse health and are crucial for synaptic plasticity events and, thereby, for learning and memory processes. Synaptic dysfunction is a common pathogenic trait of several brain disorders. In Alzheimer's Disease, the degeneration of synapses can be detected at the early stages of pathology progression before neuronal degeneration, supporting the hypothesis that synaptic failure is a major determinant of the disease. The synapse is the place where amyloid-β peptides are generated and is the target of the toxic amyloid-β oligomers. All the elements constituting the tetrapartite synapse are altered in Alzheimer's Disease and can synergistically contribute to synaptic dysfunction. Moreover, the two main hallmarks of Alzheimer's Disease, i.e., amyloid-β and tau, act in concert to cause synaptic deficits. Deciphering the mechanisms underlying synaptic dysfunction is relevant for the development of the next-generation therapeutic strategies aimed at modifying the disease progression.

Disrupted neural correlates of anesthesia and sleep reveal early circuit dysfunctions in Alzheimer models

Dysregulated homeostasis of neural activity has been hypothesized to drive Alzheimer's disease (AD) pathogenesis. AD begins with a decades-long presymptomatic phase, but whether homeostatic mechanisms already begin failing during this silent phase is unknown. We show that before the onset of memory decline and sleep disturbances, familial AD (fAD) model mice display no deficits in CA1 mean firing rate (MFR) during active wakefulness. However, homeostatic down-regulation of CA1 MFR is disrupted during non-rapid eye movement (NREM) sleep and general anesthesia in fAD mouse models. The resultant hyperexcitability is attenuated by the mitochondrial dihydroorotate dehydrogenase (DHODH) enzyme inhibitor, which tunes MFR toward lower set-point values. Ex vivo fAD mutations impair downward MFR homeostasis, resulting in pathological MFR set points in response to anesthetic drug and inhibition blockade. Thus, firing rate dyshomeostasis of hippocampal circuits is masked during active wakefulness but surfaces during low-arousal brain states, representing an early failure of the silent disease stage.

The β-Secretase Substrate Seizure 6-Like Protein (SEZ6L) Controls Motor Functions in Mice

The membrane protein seizure 6-like (SEZ6L) is a neuronal substrate of the Alzheimer's disease protease BACE1, and little is known about its physiological function in the nervous system. Here, we show that SEZ6L constitutive knockout mice display motor phenotypes in adulthood, including changes in gait and decreased motor coordination. Additionally, SEZ6L knockout mice displayed increased anxiety-like behaviour, although spatial learning and memory in the Morris water maze were normal. Analysis of the gross anatomy and proteome of the adult SEZ6L knockout cerebellum did not reveal any major differences compared to wild type, indicating that lack of SEZ6L in other regions of the nervous system may contribute to the phenotypes observed. In summary, our study establishes physiological functions for SEZ6L in regulating motor coordination and curbing anxiety-related behaviour, indicating that aberrant SEZ6L function in the human nervous system may contribute to movement disorders and neuropsychiatric diseases.

Impacts of oxidants and antioxidants on the emergence and progression of Alzheimer's disease

The brain shows a high sensitivity to oxidative stress (OS). Thus, the maintenance of homeostasis of the brain regarding the reduction-oxidation (redox) situation is crucial for the regular function of the central nervous systems (CNS). The imbalance between the reactive oxygen species (ROS) and the cellular mechanism might lead to the emergence of OS, causing profound cell death as well as tissue damages and initiating neurodegenerative disorders (NDDs). Characterized by the cytoplasmic growth of neurofibrillary tangles and extracellular β-amyloid plaques, Alzheimer's disease (AD) is a complex NDD that causes dementia in adult life with severe manifestations. Nuclear factor erythroid 2-related factor 2 (NRF2) is a key transcription factor that regulates the functional expression of OS-related genes and the functionality of endogenous antioxidants. In the case of oxidative damage, NRF2 is transferred to the nucleus and attached to the antioxidant response element (ARE) that enhances the sequence to initiate transcription of the cell-protecting genes. This review articulates various mechanisms engaged with the generation of active and reactive species of endogenous and exogenous oxidants and focuses on the antioxidants as a body defense system regarding the NRF2-ARE signaling path in the CNS.

A short peptide from sAPPα binding to BACE1-APP action site rescues Alzheimer-like pathology

Amyloid β-peptide (Aβ) is the driven force of Alzheimer's disease (AD), and reducing Aβ production could be a potential therapeutic strategy for AD. sAPPα appears to have the ability to specifically inhibit β-cleavage of APP without inhibiting BACE1 completely, direct administration of sAPPα may not be clinically applicable due to the low permeability of blood-brain barrier (BBB). In this study, we investigated the neuroprotective effects of a short peptide generated from sAPPα, which could specifically bind to BACE1 at the BACE1-APP action site. We found that this peptide significantly reduced Aβ production both in vivo and in vitro, thus further attenuated Aβ deposition, Tau hyperphosphorylation, neuroinflammation et al. and rescued behavioral deficits. Therefore, this short peptide may hold promise for the treatment of AD due to its neuroprotective effects, low molecular weight to cross BBB, and less safety concerns. The anti-neurodegenerative capacity of sAPPα may not result solely from direct inhibition of BACE1.

The promise of exosome applications in treating central nervous system diseases

Exosomes (EXs), a type of extracellular vesicles, are secreted from virtually all types of cells. EXs serve as cell-to-cell communicators by conveying proteins and nucleic acids with regulatory functions. Increasing evidence shows that EXs are implicated in the pathogenesis of central nervous system (CNS) diseases. Moreover, EXs have recently been highlighted as a new promising therapeutic strategy for in vivo delivery of nucleotides and drugs. Studies have revealed that infusion of EXs elicits beneficial effects on the CNS injury animal models. As compared to cell-based therapy, EXs-based therapy for CNS diseases has unique advantages, opening a new path for neurological medicine. In this review, we summarized the current state of knowledge of EXs, the roles and applications of EXs as a viable pathological biomarker, and EX-based therapy for CNS diseases.

The case for low-level BACE1 inhibition for the prevention of Alzheimer disease

Alzheimer disease (AD) is the most common cause of dementia in older individuals (>65 years) and has a long presymptomatic phase. Preventive therapies for AD are not yet available, and potential disease-modifying therapies targeting amyloid-β plaques in symptomatic stages of AD have only just been approved in the United States. Small-molecule inhibitors of β-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1; also known as β-secretase 1) reduce the production of amyloid-β peptide and are among the most advanced drug candidates for AD. However, to date all phase II and phase III clinical trials of BACE inhibitors were either concluded without benefit or discontinued owing to futility or the occurrence of adverse effects. Adverse effects included early, mild cognitive impairment that was associated with all but one inhibitor; preliminary results suggest that the cognitive effects are non-progressive and reversible. These discontinuations have raised questions regarding the suitability of BACE1 as a drug target for AD. In this Perspective, we discuss the status of BACE inhibitors and suggest ways in which the results of the discontinued trials can inform the development of future clinical trials of BACE inhibitors and related secretase modulators as preventative therapies. We also propose a series of experiments that should be performed to inform 'go-no-go' decisions in future trials with BACE inhibitors and consider the possibility that low levels of BACE1 inhibition could avoid adverse effects while achieving efficacy for AD prevention.

Secretases in Alzheimer's disease: Novel insights into proteolysis of APP and TREM2

Secretases are a group of proteases that are major drug targets considered for the prevention and treatment of Alzheimer's disease (AD). Secretases do not only process the AD-linked neuronal amyloid precursor protein (APP) but also the triggering receptor expressed on myeloid cells 2 (TREM2), thereby controlling microglial functions. This review highlights selected recent discoveries for the α-secretases a disintegrin and metalloprotease 10 (ADAM10) and a disintegrin and metalloprotease 17 (ADAM17), the β-secretase β-site APP cleaving enzyme 1 (BACE1) and γ-secretase and their link to AD. New genetic evidence strengthens the role of α-secretases in AD through cleavage of APP and TREM2. Novel proteins were linked to AD, which control α- and β-secretase activity through transcriptional and post-translational mechanisms. Finally, new opportunities but also challenges are discussed for pharmacologically targeting β- and γ-secretase cleavage of APP and α-secretase cleavage of TREM2 with the aim to prevent or treat AD.

Patient-specific Alzheimer-like pathology in trisomy 21 cerebral organoids reveals BACE2 as a gene dose-sensitive AD suppressor in human brain

A population of more than six million people worldwide at high risk of Alzheimer's disease (AD) are those with Down Syndrome (DS, caused by trisomy 21 (T21)), 70% of whom develop dementia during lifetime, caused by an extra copy of β-amyloid-(Aβ)-precursor-protein gene. We report AD-like pathology in cerebral organoids grown in vitro from non-invasively sampled strands of hair from 71% of DS donors. The pathology consisted of extracellular diffuse and fibrillar Aβ deposits, hyperphosphorylated/pathologically conformed Tau, and premature neuronal loss. Presence/absence of AD-like pathology was donor-specific (reproducible between individual organoids/iPSC lines/experiments). Pathology could be triggered in pathology-negative T21 organoids by CRISPR/Cas9-mediated elimination of the third copy of chromosome 21 gene BACE2, but prevented by combined chemical β and γ-secretase inhibition. We found that T21 organoids secrete increased proportions of Aβ-preventing (Aβ1-19) and Aβ-degradation products (Aβ1-20 and Aβ1-34). We show these profiles mirror in cerebrospinal fluid of people with DS. We demonstrate that this protective mechanism is mediated by BACE2-trisomy and cross-inhibited by clinically trialled BACE1 inhibitors. Combined, our data prove the physiological role of BACE2 as a dose-sensitive AD-suppressor gene, potentially explaining the dementia delay in ~30% of people with DS. We also show that DS cerebral organoids could be explored as pre-morbid AD-risk population detector and a system for hypothesis-free drug screens as well as identification of natural suppressor genes for neurodegenerative diseases.

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.

Alzheimer's Disease: A Molecular View of β-Amyloid Induced Morbific Events

Amyloid-β (Aβ) is a dynamic peptide of Alzheimer’s disease (AD) which accelerates the disease progression. At the cell membrane and cell compartments, the amyloid precursor protein (APP) undergoes amyloidogenic cleavage by β- and γ-secretases and engenders the Aβ. In addition, externally produced Aβ gets inside the cells by receptors mediated internalization. An elevated amount of Aβ yields spontaneous aggregation which causes organelles impairment. Aβ stimulates the hyperphosphorylation of tau protein via acceleration by several kinases. Aβ travels to the mitochondria and interacts with its functional complexes, which impairs the mitochondrial function leading to the activation of apoptotic signaling cascade. Aβ disrupts the Ca²⁺ and protein homeostasis of the endoplasmic reticulum (ER) and Golgi complex (GC) that promotes the organelle stress and inhibits its stress recovery machinery such as unfolded protein response (UPR) and ER-associated degradation (ERAD). At lysosome, Aβ precedes autophagy dysfunction upon interacting with autophagy molecules. Interestingly, Aβ act as a transcription regulator as well as inhibits telomerase activity. Both Aβ and p-tau interaction with neuronal and glial receptors elevate the inflammatory molecules and persuade inflammation. Here, we have expounded the Aβ mediated events in the cells and its cosmopolitan role on neurodegeneration, and the current clinical status of anti-amyloid therapy.

The Uppsala APP deletion causes early onset autosomal dominant Alzheimer's disease by altering APP processing and increasing amyloid β fibril formation

Point mutations in the amyloid precursor protein gene (APP) cause familial Alzheimer's disease (AD) by increasing generation or altering conformation of amyloid β (Aβ). Here, we describe the Uppsala APP mutation (Δ690-695), the first reported deletion causing autosomal dominant AD. Affected individuals have an age at symptom onset in their early forties and suffer from a rapidly progressing disease course. Symptoms and biomarkers are typical of AD, with the exception of normal cerebrospinal fluid (CSF) Aβ42 and only slightly pathological amyloid-positron emission tomography signals. Mass spectrometry and Western blot analyses of patient CSF and media from experimental cell cultures indicate that the Uppsala APP mutation alters APP processing by increasing β-secretase cleavage and affecting α-secretase cleavage. Furthermore, in vitro aggregation studies and analyses of patient brain tissue samples indicate that the longer form of mutated Aβ, AβUpp1-42_Δ19-24, accelerates the formation of fibrils with unique polymorphs and their deposition into amyloid plaques in the affected brain.

Functional Neurophysiological Biomarkers of Early-Stage Alzheimer's Disease: A Perspective of Network Hyperexcitability in Disease Progression

Network hyperexcitability (NH) has recently been suggested as a potential neurophysiological indicator of Alzheimer’s disease (AD), as new, more accurate biomarkers of AD are sought. NH has generated interest as a potential indicator of certain stages in the disease trajectory and even as a disease mechanism by which network dysfunction could be modulated. NH has been demonstrated in several animal models of AD pathology and multiple lines of evidence point to the existence of NH in patients with AD, strongly supporting the physiological and clinical relevance of this readout. Several hypotheses have been put forward to explain the prevalence of NH in animal models through neurophysiological, biochemical, and imaging techniques. However, some of these hypotheses have been built on animal models with limitations and caveats that may have derived NH through other mechanisms or mechanisms without translational validity to sporadic AD patients, potentially leading to an erroneous conclusion of the underlying cause of NH occurring in patients with AD. In this review, we discuss the substantiation for NH in animal models of AD pathology and in human patients, as well as some of the hypotheses considering recently developed animal models that challenge existing hypotheses and mechanisms of NH. In addition, we provide a preclinical perspective on how the development of animal models incorporating AD-specific NH could provide physiologically relevant translational experimental data that may potentially aid the discovery and development of novel therapies for AD.