BACE2, as a novel APP theta-secretase, is not responsible for the pathogenesis of Alzheimer's disease in Down syndrome

Deciphering the role of APOE in cerebral amyloid angiopathy: from genetic insights to therapeutic horizons

Cerebral amyloid angiopathy (CAA), characterized by the deposition of amyloid-β (Aβ) peptides in the walls of medium and small vessels of the brain and leptomeninges, is a major cause of lobar hemorrhage in elderly individuals. Among the genetic risk factors for CAA that continue to be recognized, the apolipoprotein E (APOE) gene is the most significant and prevalent, as its variants have been implicated in more than half of all patients with CAA. While the presence of the APOE ε4 allele markedly increases the risk of CAA, the ε2 allele confers a protective effect relative to the common ε3 allele. These allelic variants encode three APOE isoforms that differ at two amino acid positions. The primary physiological role of APOE is to mediate lipid transport in the brain and periphery; however, it has also been shown to be involved in a wide array of biological functions, particularly those involving Aβ, in which it plays a known role in processing, production, aggregation, and clearance. The challenges posed by the reliance on postmortem histological analyses and the current absence of an effective intervention underscore the urgency for innovative APOE-targeted strategies for diagnosing CAA. This review not only deepens our understanding of the impact of APOE on the pathogenesis of CAA but can also help guide the exploration of targeted therapies, inspiring further research into the therapeutic potential of APOE.

Updates in Alzheimer's disease: from basic research to diagnosis and therapies

Alzheimer's disease (AD) is the most common neurodegenerative disorder, characterized pathologically by extracellular deposition of β-amyloid (Aβ) into senile plaques and intracellular accumulation of hyperphosphorylated tau (pTau) as neurofibrillary tangles. Clinically, AD patients show memory deterioration with varying cognitive dysfunctions. The exact molecular mechanisms underlying AD are still not fully understood, and there are no efficient drugs to stop or reverse the disease progression. In this review, we first provide an update on how the risk factors, including APOE variants, infections and inflammation, contribute to AD; how Aβ and tau become abnormally accumulated and how this accumulation plays a role in AD neurodegeneration. Then we summarize the commonly used experimental models, diagnostic and prediction strategies, and advances in periphery biomarkers from high-risk populations for AD. Finally, we introduce current status of development of disease-modifying drugs, including the newly officially approved Aβ vaccines, as well as novel and promising strategies to target the abnormal pTau. Together, this paper was aimed to update AD research progress from fundamental mechanisms to the clinical diagnosis and therapies.

It's good to know what to BACE the specificity of your inhibitors on

Production, aggregation, and clearance of the amyloid β peptide (Aβ) are important processes governing the initial pathogenesis of Alzheimer's disease (AD). Inhibition of β-site amyloid precursor protein (APP) cleaving enzyme (BACE1) (one of two key proteases responsible for Aβ production) as an AD-therapeutic approach so far has failed to yield a successful drug. BACE1 and its homologue BACE2 are frequently inhibited by the same inhibitors. Several genetic and cerebral organoid modeling studies suggest that BACE2 has dose-dependent AD-suppressing activity, which makes its unwanted inhibition potentially counterproductive for AD treatment. The in vivo effects of an unwanted cross inhibition of BACE2 have so far been impossible to monitor because of the lack of an easily accessible pharmacodynamic marker specific for BACE2 cleavage. In this issue of the JCI, work led by Stefan F. Lichtenthaler identifies soluble VEGFR3 (sVEGFR3) as a pharmacodynamic plasma marker for BACE2 activity not shared with BACE1.

Analysis of microisolated frontal cortex excitatory layer III and V pyramidal neurons reveals a neurodegenerative phenotype in individuals with Down syndrome

We elucidated the molecular fingerprint of vulnerable excitatory neurons within select cortical lamina of individuals with Down syndrome (DS) for mechanistic understanding and therapeutic potential that also informs Alzheimer's disease (AD) pathophysiology. Frontal cortex (BA9) layer III (L3) and layer V (L5) pyramidal neurons were microisolated from postmortem human DS and age- and sex-matched controls (CTR) to interrogate differentially expressed genes (DEGs) and key biological pathways relevant to neurodegenerative programs. We identified > 2300 DEGs exhibiting convergent dysregulation of gene expression in both L3 and L5 pyramidal neurons in individuals with DS versus CTR subjects. DEGs included over 100 triplicated human chromosome 21 genes in L3 and L5 neurons, demonstrating a trisomic neuronal karyotype in both laminae. In addition, thousands of other DEGs were identified, indicating gene dysregulation is not limited to trisomic genes in the aged DS brain, which we postulate is relevant to AD pathobiology. Convergent L3 and L5 DEGs highlighted pertinent biological pathways and identified key pathway-associated targets likely underlying corticocortical neurodegeneration and related cognitive decline in individuals with DS. Select key DEGs were interrogated as potential hub genes driving dysregulation, namely the triplicated DEGs amyloid precursor protein (APP) and superoxide dismutase 1 (SOD1), along with key signaling DEGs including mitogen activated protein kinase 1 and 3 (MAPK1, MAPK3) and calcium calmodulin dependent protein kinase II alpha (CAMK2A), among others. Hub DEGs determined from multiple pathway analyses identified potential therapeutic candidates for amelioration of cortical neuron dysfunction and cognitive decline in DS with translational relevance to AD.

The Interplay of Protein Aggregation, Genetics, and Oxidative Stress in Alzheimer's Disease: Role for Natural Antioxidants and Immunotherapeutics

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that comprises amyloid-beta protein (Aβ) as a main component of neuritic plaques. Its deposition is considered a trigger for AD pathogenesis, progression, and the clinical symptoms of cognitive impairment. Some distinct pathological features of AD include phosphorylation of tau protein, oxidative stress, and mitochondrial dysfunction. These pathological consequences tend to produce reactive oxygen species (ROS), resulting in the dysregulation of various signaling pathways of neuroinflammation and neurodegeneration. The relationship between the Aβ cascade and oxidative stress in AD pathogenesis is like a "chicken and egg" story, with the etiology of the disease regarding these two factors remaining a question of "which comes first." However, in this review, we have tried our best to clarify the interconnection between these two mechanisms and to show the precise cause-and-effect relationship. Based on the above hallmarks of AD, several therapeutic strategies using natural antioxidants, monoclonal antibodies, and vaccines are employed as anti-Aβ therapy to decrease ROS, Aβ burden, chronic neuroinflammation, and synaptic failure. These natural antioxidants and immunotherapeutics have demonstrated significant neuroprotective effects and symptomatic relief in various in vitro and in vivo models, as well as in clinical trials for AD. However, none of them have received final approval to enter the drug market for mitigating AD. In this review, we extensively elaborate on the pitfalls, assurances, and important crosstalk between oxidative stress and Aβ concerning current anti-Aβ therapy. Additionally, we discuss future strategies for the development of more Aβ-targeted approaches and the optimization of AD treatment and mitigation.

ICA1 affects APP processing through the PICK1-PKCα signaling pathway

AIMS

Islet cell autoantigen 1 (ICA1) is involved in autoimmune diseases and may affect synaptic plasticity as a neurotransmitter. Databases related to Alzheimer's disease (AD) have shown decreased ICA1 expression in patients with AD. However, the role of ICA1 in AD remains unclear. Here, we report that ICA1 expression is decreased in the brains of patients with AD and an AD mouse model.

RESULTS

The ICA1 increased the expression of amyloid precursor protein (APP), disintegrin and metalloprotease 10 (ADAM10), and disintegrin and metalloprotease 17 (ADAM17), but did not affect protein half-life or mRNA levels. Transcriptome sequencing analysis showed that ICA1 regulates the G protein-coupled receptor signaling pathway. The overexpression of ICA1 increased PKCα protein levels and phosphorylation.

CONCLUSION

Our results demonstrated that ICA1 shifts APP processing to non-amyloid pathways by regulating the PICK1-PKCα signaling pathway. Thus, this study suggests that ICA1 is a novel target for the treatment of AD.

Early Animal Origin of BACE1 APP/Aβ Proteolytic Function

Alzheimer's disease is characterized, in part, by the accumulation of β-amyloid (Aβ) in the brain. Aβ is produced via the proteolysis of APP by BACE1 and γ-secretase. Since BACE1 is the rate-limiting enzyme in the production of Aβ, and a target for therapeutics, it is of interest to know when its proteolytic function evolved and for what purpose. Here, we take a functional evolutionary approach to show that BACE1 likely evolved from a gene duplication event near the base of the animal clade and that BACE1 APP/Aβ proteolytic function evolved during early animal diversification, hundreds of millions of years before the evolution of the APP/Aβ substrate. Our examination of BACE1 APP/Aβ proteolytic function includes cnidarians, ctenophores, and choanoflagellates. The most basal BACE1 ortholog is found in cnidarians, while ctenophores, placozoa, and choanoflagellates have genes equally orthologous to BACE1 and BACE2. BACE1 from a cnidarian (Hydra) can cleave APP to release Aβ, pushing back the date of the origin of its function to near the origin of animals. We tested more divergent BACE1/2 genes from a ctenophore (Mnemiopsis) and a choanoflagellate (Monosiga), and neither has this activity. These findings indicate that the specific proteolytic function of BACE1 evolved during the very earliest diversification of animals, most likely after a gene-duplication event.

A Strategy for Allowing Earlier Diagnosis and Rigorous Evaluation of BACE1 Inhibitors in Preclinical Alzheimer's Disease

Given continued failure of BACE1 inhibitor programs at symptomatic and prodromal stages of Alzheimer's disease (AD), clinical trials need to target the earlier preclinical stage. However, trial design is complex in this population with negative diagnosis of classical hippocampal amnesia on standard memory tests. Besides recent advances in brain imaging, electroencephalogram, and fluid-based biomarkers, new cognitive markers should be established for earlier diagnosis that can optimize recruitment to BACE1 inhibitor trials in presymptomatic AD. Notably, accelerated long-term forgetting (ALF) is emerging as a sensitive cognitive measure that can discriminate between asymptomatic individuals with high risks for developing AD and healthy controls. ALF is a form of declarative memory impairment characterized by increased forgetting rates over longer delays (days to months) despite normal storage within the standard delays of testing (20-60 min). Therefore, ALF may represent a harbinger of preclinical dementia and the impairment of systems memory consolidation, during which memory traces temporarily stored in the hippocampus become gradually integrated into cortical networks. This review provides an overview of the utility of ALF in a rational design of next-generation BACE1 inhibitor trials in preclinical AD. I explore potential mechanisms underlying ALF and relevant early-stage biomarkers useful for BACE1 inhibitor evaluation, including synaptic protein alterations, astrocytic dysregulation and neuron hyperactivity in the hippocampal-cortical network. Furthermore, given the physiological role of the isoform BACE2 as an AD-suppressor gene, I also discuss the possible association between the poor selectivity of BACE1 inhibitors and their side effects (e.g., cognitive worsening) in prior clinical trials.

BACE2 deficiency impairs expression and function of endothelial nitric oxide synthase in brain endothelial cells

Beta-site amyloid precursor protein (APP)-cleaving enzyme 2 (BACE2) is highly expressed in cerebrovascular endothelium. Notably, BACE2 is one of the most downregulated genes in cerebrovascular endothelium derived from patients with Alzheimer's disease. The present study was designed to determine the role of BACE2 in control of expression and function of endothelial nitric oxide synthase (eNOS). Genetic downregulation of BACE2 with small interfering RNA (BACE2siRNA) in human brain microvascular endothelial cells (BMECs) significantly decreased expression of eNOS and elevated levels of eNOS phosphorylated at threonine residue Thr495, thus leading to reduced production of nitric oxide (NO). BACE2siRNA also suppressed expression of APP and decreased production and release of soluble APPα (sAPPα). In contrast, adenovirus-mediated overexpression of APP increased expression of eNOS. Consistent with these observations, nanomolar concentrations of sAPPα and APP 17mer peptide (derived from sAPPα) augmented eNOS expression. Further analysis established that γ-aminobutyric acid type B receptor subunit 1 and Krüppel-like factor 2 may function as downstream molecular targets significantly contributing to BACE2/APP/sAPPα-induced up-regulation of eNOS. In agreement with studies on cultured human endothelium, endothelium-dependent relaxations to acetylcholine and basal production of cyclic GMP were impaired in cerebral arteries of BACE2-deficient mice. We propose that in the brain blood vessels, BACE2 may function as a vascular protective protein.

Amyloid β-based therapy for Alzheimer's disease: challenges, successes and future

Amyloid β protein (Aβ) is the main component of neuritic plaques in Alzheimer's disease (AD), and its accumulation has been considered as the molecular driver of Alzheimer's pathogenesis and progression. Aβ has been the prime target for the development of AD therapy. However, the repeated failures of Aβ-targeted clinical trials have cast considerable doubt on the amyloid cascade hypothesis and whether the development of Alzheimer's drug has followed the correct course. However, the recent successes of Aβ targeted trials have assuaged those doubts. In this review, we discussed the evolution of the amyloid cascade hypothesis over the last 30 years and summarized its application in Alzheimer's diagnosis and modification. In particular, we extensively discussed the pitfalls, promises and important unanswered questions regarding the current anti-Aβ therapy, as well as strategies for further study and development of more feasible Aβ-targeted approaches in the optimization of AD prevention and treatment.

PCP4 Promotes Alzheimer's Disease Pathogenesis by Affecting Amyloid-β Protein Precursor Processing

BACKGROUND

Down syndrome (DS) is caused by an extra copy of all or part of chromosome 21. The patients with DS develop typical Alzheimer's disease (AD) neuropathology, indicating the role of genes on human chromosome 21 (HSA21) in the pathogenesis of AD. Purkinje cell protein 4 (PCP4), also known as brain-specific protein 19, is a critical gene located on HSA21. However, the role of PCP4 in DS and AD pathogenesis is not clear.

OBJECTIVE

To explore the role of PCP4 in amyloid-β protein precursor (AβPP) processing in AD.

METHODS

In this study, we investigated the role of PCP4 in AD progression in vitro and in vivo. In vitro experiments, we overexpressed PCP4 in human Swedish mutant AβPP stable expression or neural cell lines. In vitro experiments, APP23/PS45 double transgenic mice were selected and treated with AAV-PCP4. Multiple topics were detected by western blot, RT-PCR, immunohistochemical and behavioral test.

RESULTS

We found that PCP4 expression was altered in AD. PCP4 was overexpressed in APP23/PS45 transgenic mice and PCP4 affected the processing of AβPP. The production of amyloid-β protein (Aβ) was also promoted by PCP4. The upregulation of endogenous AβPP expression and the downregulation of ADAM10 were due to the transcriptional regulation of PCP4. In addition, PCP4 increased Aβ deposition and neural plaque formation in the brain, and exuberated learning and memory impairment in transgenic AD model mice.

CONCLUSION

Our finding reveals that PCP4 contributes to the pathogenesis of AD by affecting AβPP processing and suggests PCP4 as a novel therapeutic target for AD by targeting Aβ pathology.

USP25 contributes to defective neurogenesis and cognitive impairments

Both Down syndrome (DS) individuals and animal models exhibit hypo-cellularity in hippocampus and neocortex indicated by enhanced neuronal death and compromised neurogenesis. Ubiquitin-specific peptidase 25 (USP25), a human chromosome 21 (HSA21) gene, encodes for a deubiquitinating enzyme overexpressed in DS patients. Dysregulation of USP25 has been associated with Alzheimer's phenotypes in DS, but its role in defective neurogenesis in DS has not been defined. In this study, we found that USP25 upregulation impaired cell cycle regulation during embryonic neurogenesis and cortical development. Overexpression of USP25 in hippocampus promoted the neural stem cells to glial cell fates and suppressed neuronal cell fate by altering the balance between cyclin D1 and cyclin D2, thus reducing neurogenesis in the hippocampus. USP25-Tg mice showed increased anxiety/depression-like behaviors and learning and memory deficits. These results suggested that USP25 overexpression resulted in defective neurogenesis and cognitive impairments, which could contribute to the pathogenesis of DS. USP25 may be a potential pharmaceutical target for DS.

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.

BACE2: A Promising Neuroprotective Candidate for Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia that affects millions of predominantly elderly individuals worldwide. Despite intensive research over several decades, controversies still surround the etiology of AD and the disease remains incurable. Meanwhile, new molecular players of the central amyloid cascade hypothesis have emerged and among these is a protease known as β-site APP cleavage enzyme 2 (BACE2). Unlike BACE1, BACE2 cleaves the amyloid-β protein precursor within the Aβ domain that accordingly prevents the generation of Aβ42 peptides, the aggregation of which is commonly regarded as the toxic entity that drives neurodegeneration in AD. Given this non-amyloidogenic role of BACE2, it is attractive to position BACE2 as a therapeutic target for AD. Indeed, several groups including ours have demonstrated a neuroprotective role for BACE2 in AD. In this review, we discuss emerging evidence supporting the ability of BACE2 in mitigating AD-associated pathology in various experimental systems including human pluripotent stem cell-derived cerebral organoid disease models. Alongside this, we also provide an update on the identification of single nucleotide polymorphisms occurring in the BACE2 gene that are linked to increased risk and earlier disease onset in the general population. In particular, we highlight a recently identified point mutation on BACE2 that apparently leads to sporadic early-onset AD. We believe that a better understanding of the role of BACE2 in AD would provide new insights for the development of viable therapeutic strategies for individuals with dementia.

Biological Mechanism-based Neurology and Psychiatry: A BACE1/2 and Downstream Pathway Model

In oncology, comprehensive omics and functional enrichment studies have led to an extensive profiling of (epi)genetic and neurobiological alterations that can be mapped onto a single tumor's clinical phenotype and divergent clinical phenotypes expressing common pathophysiological pathways. Consequently, molecular pathway-based therapeutic interventions for different cancer typologies, namely tumor type- and site-agnostic treatments, have been developed, encouraging the real-world implementation of a paradigm shift in medicine. Given the breakthrough nature of the new-generation translational research and drug development in oncology, there is an increasing rationale to transfertilize this blueprint to other medical fields, including psychiatry and neurology. In order to illustrate the emerging paradigm shift in neuroscience, we provide a state-of-the-art review of translational studies on the β-site amyloid precursor protein cleaving enzyme (BACE) and its most studied downstream effector, neuregulin, which are molecular orchestrators of distinct biological pathways involved in several neurological and psychiatric diseases. This body of data aligns with the evidence of a shared genetic/biological architecture among Alzheimer's disease, schizoaffective disorder, and autism spectrum disorders. To facilitate a forward-looking discussion about a potential first step towards the adoption of biological pathway-based, clinical symptom-agnostic, categorization models in clinical neurology and psychiatry for precision medicine solutions, we engage in a speculative intellectual exercise gravitating around BACE-related science, which is used as a paradigmatic case here. We draw a perspective whereby pathway-based therapeutic strategies could be catalyzed by highthroughput techniques embedded in systems-scaled biology, neuroscience, and pharmacology approaches that will help overcome the constraints of traditional descriptive clinical symptom and syndrome-focused constructs in neurology and psychiatry.

Epstein-Barr Virus-Induced Genes and Endogenous Retroviruses in Immortalized B Cells from Patients with Multiple Sclerosis

The immune pathogenesis of multiple sclerosis (MS) is thought to be triggered by environmental factors in individuals with an unfavorable genetic predisposition. Epstein-Barr virus (EBV) infection is a major risk factor for subsequent development of MS. Human endogenous retroviruses (HERVs) can be activated by EBV, and might be a missing link between an initial EBV infection and the later onset of MS. In this study, we investigated differential gene expression patterns in EBV-immortalized lymphoblastoid B cell lines (LCL) from MS-affected individuals (MSLCL) and controls by using RNAseq and qRT-PCR. RNAseq data from LCL mapped to the human genome and a virtual virus metagenome were used to identify possible biomarkers for MS or disease-relevant risk factors, e.g., the relapse rate. We observed that lytic EBNA-1 transcripts seemed to be negatively correlated with age leading to an increased expression in LCL from younger PBMC donors. Further, HERV-K (HML-2) GAG was increased upon EBV-triggered immortalization. Besides the well-known transactivation of HERV-K18, our results suggest that another six HERV loci are up-regulated upon stimulation with EBV. We identified differentially expressed genes in MSLCL, e.g., several HERV-K loci, ERVMER61-1 and ERV3-1, as well as genes associated with relapses. In summary, EBV induces genes and HERV in LCL that might be suitable as biomarkers for MS or the relapse risk.

Vav2 is a novel APP-interacting protein that regulates APP protein level

Amyloid precursor protein (APP) is a transmembrane protein that plays critical role in the pathogenesis of Alzheimer's disease (AD). It is also involved in many types of cancers. Increasing evidence has shown that the tyrosine phosphorylation site Y682 in the intracellular tail of APP is crucial for APP function. Here, we report that Vav2, a guanine nucleotide exchange factor (GEF) for Rho family GTPase, is a novel interaction partner of APP. We found that Vav2-SH2 domain was able to bind directly to the Y682-phosphorylated intracellular tail of APP through isothermal titration calorimetry and NMR titrating experiments. The crystal structure of Vav2-SH2 in complex with an APP-derived phosphopeptide was determined to understand the structural basis of this recognition specificity. The interaction of APP and Vav2 in a full-length manner was further confirmed in cells by GST pull-down, co-immunoprecipitation and immunofluorescence staining experiments. In addition, we found overexpression of Vav2 could inhibit APP degradation and markedly increase the protein levels of APP and its cleavage productions in 20E2 cells, and this function of Vav2 required a functional SH2 domain.

Amylin and Secretases in the Pathology and Treatment of Alzheimer's Disease

Alzheimer's disease remains a prevailing neurodegenerative condition which has an array physical, emotional, and financial consequences to patients and society. In the past decade, there has been a greater degree of investigation on therapeutic small peptides. This group of biomolecules have a profile of fundamentally sound characteristics which make them an intriguing area for drug development. Among these biomolecules, there are four modulatory mechanisms of interest in this review: alpha-, beta-, gamma-secretases, and amylin. These protease-based biomolecules all have a contributory role in the amyloid cascade hypothesis. Moreover, the involvement of various biochemical pathways intertwines these peptides to have shared regulators (i.e., retinoids). Further clinical and translational investigation must occur to gain a greater understanding of its potential application in patient care. The aim of this narrative review is to evaluate the contemporary literature on these protease biomolecule modulators and determine its utility in the treatment of Alzheimer's disease.

USP25 inhibition ameliorates Alzheimer's pathology through the regulation of APP processing and Aβ generation

Down syndrome (DS), or trisomy 21, is one of the critical risk factors for early-onset Alzheimer’s disease (AD), implicating key roles for chromosome 21–encoded genes in the pathogenesis of AD. We previously identified a role for the deubiquitinase USP25, encoded on chromosome 21, in regulating microglial homeostasis in the AD brain; however, whether USP25 affects amyloid pathology remains unknown. Here, by crossing 5×FAD AD and Dp16 DS mice, we observed that trisomy 21 exacerbated amyloid pathology in the 5×FAD brain. Moreover, bacterial artificial chromosome (BAC) transgene–mediated USP25 overexpression increased amyloid deposition in the 5×FAD mouse brain, whereas genetic deletion of Usp25 reduced amyloid deposition. Furthermore, our results demonstrate that USP25 promoted β cleavage of APP and Aβ generation by reducing the ubiquitination and lysosomal degradation of both APP and BACE1. Importantly, pharmacological inhibition of USP25 ameliorated amyloid pathology in the 5×FAD mouse brain. In summary, we identified the DS-related gene USP25 as a critical regulator of AD pathology, and our data suggest that USP25 serves as a potential pharmacological target for AD drug development.

Revisiting APP secretases: an overview on the holistic effects of retinoic acid receptor stimulation in APP processing

Alzheimer's disease (AD) is the leading cause of dementia worldwide and is characterized by the accumulation of the β-amyloid peptide (Aβ) in the brain, along with profound alterations in phosphorylation-related events and regulatory pathways. The production of the neurotoxic Aβ peptide via amyloid precursor protein (APP) proteolysis is a crucial step in AD development. APP is highly expressed in the brain and is complexly metabolized by a series of sequential secretases, commonly denoted the α-, β-, and γ-cleavages. The toxicity of resulting fragments is a direct consequence of the first cleaving event. β-secretase (BACE1) induces amyloidogenic cleavages, while α-secretases (ADAM10 and ADAM17) result in less pathological peptides. Hence this first cleavage event is a prime therapeutic target for preventing or reverting initial biochemical events involved in AD. The subsequent cleavage by γ-secretase has a reduced impact on Aβ formation but affects the peptides' aggregating capacity. An array of therapeutic strategies are being explored, among them targeting Retinoic Acid (RA) signalling, which has long been associated with neuronal health. Additionally, several studies have described altered RA levels in AD patients, reinforcing RA Receptor (RAR) signalling as a promising therapeutic strategy. In this review we provide a holistic approach focussing on the effects of isoform-specific RAR modulation with respect to APP secretases and discuss its advantages and drawbacks in subcellular AD related events.

The Clinical and Neuropathological Features of Sporadic (Late-Onset) and Genetic Forms of Alzheimer's Disease

The purpose of this review is to compare and highlight the clinical and pathological aspects of genetic versus acquired Alzheimer's disease: Down syndrome-associated Alzheimer's disease in (DSAD) and Autosomal Dominant Alzheimer's disease (ADAD) are compared with the late-onset form of the disease (LOAD). DSAD and ADAD present in a younger population and are more likely to manifest with non-amnestic (such as dysexecutive function features) in the prodromal phase or neurological features (such as seizures and paralysis) especially in ADAD. The very large variety of mutations associated with ADAD explains the wider range of phenotypes. In the LOAD, age-associated comorbidities explain many of the phenotypic differences.

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.

Selective Secretase Targeting for Alzheimer's Disease Therapy

Alzheimer's disease (AD) is associated with marked atrophy of the cerebral cortex and accumulation of amyloid plaques and neurofibrillary tangles. Amyloid plaques are formed by oligomers of amyloid-β (Aβ) in the brain, with a length of 42 and 40 amino acids. α-secretase cleaves amyloid-β protein precursor (AβPP) producing the membrane-bound fragment CTFα and the soluble fragment sAβPPα with neuroprotective activity; β-secretase produces membrane-bound fragment CTFβ and a soluble fragment sAβPPβ. After α-secretase cleavage of AβPP, γ-secretase cleaves CTFα to produce the cytoplasmic fragment AICD and P3 in the non-amyloidogenic pathway. CTFβ is cleaved by γ-secretase producing AICD as well as Aβ in amyloidogenic pathways. In the last years, the study of natural products and synthetic compounds, such as α-secretase activity enhancers, β-secretase inhibitors (BACE-1), and γ-secretase activity modulators, have been the focus of pharmaceuticals and researchers. Drugs were improved regarding solubility, blood-brain barrier penetration, selectivity, and potency decreasing Aβ42. In this regard, BACE-1 inhibitors, such as Atabecestat, NB-360, Umibecestat, PF-06751979 Verubecestat, LY2886721, Lanabecestat, LY2811376 and Elenbecestat, were submitted to phase I-III clinical trials. However, inhibition of Aβ production did not recover cognitive functions or reverse disease progress. Novel strategies are being developed, aiming at a partial reduction of Aβ production, such as the development of γ-secretase modulators or α-secretase activity enhancers. Such therapeutic tools shall focus on slowing down or minimizing the progression of neuronal damage. Here, we summarize structures and activities of the latest compounds designed for AD treatment, with remarkable in vitro, in vivo, and clinical phase activities.

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.

Ginkgolide B‑induced AMPK pathway activation protects astrocytes by regulating endoplasmic reticulum stress, oxidative stress and energy metabolism induced by Aβ1‑42

Ginkgolide B (GB), the diterpenoid lactone compound isolated from the extracts of Ginkgo biloba leaves, significantly improves cognitive impairment, but its potential pharmacological effect on astrocytes induced by β-amyloid (Aβ)_1-42 remains to be elucidated. The present study aimed to investigate the protective effect and mechanism of GB on astrocytes with Aβ_1-42-induced apoptosis in Alzheimer's disease (AD). Astrocytes obtained from Sprague Dawley rats were randomly divided into control, Aβ, GB and GB + compound C groups. Cell viability and apoptosis were analyzed using Cell Counting Kit-8 and flow cytometry assays, respectively. Protein and mRNA expression levels were analyzed using western blotting and reverse transcription-quantitative PCR, respectively. The levels of superoxide dismutase (SOD), malondialdehyde (MDA), glutathione peroxidase (GSH-Px), reactive oxygen species (ROS) and ATP were determined using the corresponding commercial kits. The findings revealed that GB attenuated Aβ_1-42-induced apoptosis and the 5′ adenosine monophosphate- activated protein kinase (AMPK) inhibitor compound C reversed the protective effects of GB. In addition, GB reversed Aβ_1-42-induced oxidative damage and energy metabolism disorders, including decreases in the levels of SOD, GSH-Px and ATP and increased the levels of MDA and ROS in astrocytes, while compound C reversed the anti-oxidative effect and the involvement of GB in maintaining energy metabolism in astrocytes. Finally, GB decreased the expression levels of the endoplasmic reticulum stress (ERS) proteins and the apoptotic protein CHOP and increased both mRNA and protein expression of the components of the energy metabolism-related AMPK/peroxisome proliferator-activated receptor γ coactivator 1α/peroxisome proliferator-activated receptor α and anti-oxidation-related nuclear respiratory factor 2/heme oxygenase 1/NAD(P)H dehydrogenase (quinone 1) pathways and downregulated the expression of β-secretase 1. However, compound C could antagonize these effects. In conclusion, the findings demonstrated that GB protected against Aβ_1-42-induced apoptosis by inhibiting ERS, oxidative stress, energy metabolism disorders and Aβ_1-42 production probably by activating AMPK signaling pathways. The findings provided an innovative insight into the treatment using GB as a therapeutic in Aβ_1-42-related AD.

Characterization of monomeric and soluble aggregated Aβ in Down's syndrome and Alzheimer's disease brains

The major characteristics of Alzheimer's disease (AD) are amyloid plaques, consisting of aggregated beta amyloid (Aβ) peptides, together with tau pathology (tangles, neuropil treads and dystrophic neurites surrounding the plaques), in the brain. Down's syndrome (DS) individuals are at increased risk to develop AD-type pathology; most DS individuals have developed substantial pathology already at the age of 40. DS individuals have an extra copy of chromosome 21, harbouring the amyloid precursor protein gene (APP). Our aim was to investigate the Aβ peptide pattern in DS and AD brains to investigate differences in their amyloid deposition and aggregation, respectively. Cortical tissue from patients with DS (with amyloid pathology), sporadic AD and controls were homogenized and fractionated into TBS (water soluble) and formic acid (water insoluble) fractions. Immunoprecipitation (IP) was performed using a variety of antibodies targeting different Aβ species including oligomeric Aβ. Mass spectrometry was then used to evaluate the presence of Aβ species in the different patient groups. A large number of Aβ peptides were identified including Aβ1-X, 2-X, 3-X, 4-X, 5-X, 11-X, and Aβ peptides extended N terminally of the BACE1 cleavage site and ending at amino 15 in the Aβ sequence APP/Aβ(-X to 15), as well as peptides post-translationally modified by pyroglutamate formation. Most Aβ peptides had higher abundance in AD and DS compared to controls, except the APP/Aβ(-X to 15) peptides which were most abundant in DS followed by controls and AD. Furthermore, the abundancies of AβX-40 and AβX-34 were increased in DS compared with AD. Aβ1-40, Aβ1-42, and Aβ4-42 were identified as the main constitutes of protofibrils (IP'd using mAb158) and higher relative Aβ1-42 signals were obtained compared with samples IP'd with 6E10 + 4G8, indicating that the protofibrils/oligomers were enriched with peptides ending at amino acid 42. All Aβ peptides found in AD were also present in DS indicating similar pathways of Aβ peptide production, degradation and accumulation, except for APP/Aβ(-X to 15). Likewise, the Aβ peptides forming protofibrils/oligomers in both AD and DS were similar, implying the possibility that treatment with clinical benefit in sporadic AD might also be beneficial for subjects with DS.

The Potential Role of Ferroptosis in Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent cause of dementia, accounting for approximately 60%-80%of all cases. Although much effort has been made over the years, the precise mechanism of AD has not been completely elucidated. Recently, great attention has shifted to the roles of iron metabolism, lipid peroxidation, and oxidative stress in AD pathogenesis. We also note that these pathological events are the vital regulators of a novel regulatory cell death, termed ferroptosis-an iron-dependent, oxidative, non-apoptotic cell death. Ferroptosis differs from apoptosis, necrosis, and autophagy with respect to morphology, biochemistry, and genetics. Mounting evidence suggests that ferroptosis may be involved in neurological disorders, including AD. Here, we review the underlying mechanisms of ferroptosis; discuss the potential interaction between AD and ferroptosis in terms of iron metabolism, lipid peroxidation, and the glutathione/glutathione peroxidase 4 axis; and describe some associated studies that have explored the implication of ferroptosis in AD.

An empirical bayesian approach for testing gene expression fold change and its application in detecting global dosage effects

We are motivated by biological studies intended to understand global gene expression fold change. Biologists have generally adopted a fixed cutoff to determine the significance of fold changes in gene expression studies (e.g. by using an observed fold change equal to two as a fixed threshold). Scientists can also use a t-test or a modified differential expression test to assess the significance of fold changes. However, these methods either fail to take advantage of the high dimensionality of gene expression data or fail to test fold change directly. Our research develops a new empirical Bayesian approach to substantially improve the power and accuracy of fold-change detection. Specifically, we more accurately estimate gene-wise error variation in the log of fold change. We then adopt a t-test with adjusted degrees of freedom for significance assessment. We apply our method to a dosage study in Arabidopsis and a Down syndrome study in humans to illustrate the utility of our approach. We also present a simulation study based on real datasets to demonstrate the accuracy of our method relative to error variance estimation and power in fold-change detection. Our developed R package with a detailed user manual is publicly available on GitHub at https://github.com/cuiyingbeicheng/Foldseq.

Advances in Drug Therapy for Alzheimer's Disease

Alzheimer's disease (AD) is a chronic neurodegenerative disease that mainly causes dementia. It is a serious threat to the health of the global elderly population. Considerable money and effort has been invested in the development of drug therapy for AD worldwide. Many drug therapies are currently under development or in clinical trials, based on two known mechanisms of AD, namely, Aβ toxicity and the abnormal Tau hyperphosphorylation. Numerous drugs are also being developed for other AD associated mechanisms such as neuroinflammation, neurotransmitter imbalance, oxidative damage and mitochondrial dysfunction, neuron loss and degeneration. Even so, the number of drugs that can successfully improve symptoms or delay the progression of the disease remains very limited. However, multi-drug combinations may provide a new avenue for drug therapy for AD. In addition, early diagnosis of AD and timely initiation of treatment may allow drugs that act on the early pathological processes of AD to help improve the symptoms and prevent the progression of the condition.

Is It the Twilight of BACE1 Inhibitors?

β-secretase (BACE1) has been regarded as a prime target for the development of amyloid beta (Aβ) lowering drugs in the therapy of Alzheimer´s disease (AD). Although the enzyme was discovered in 1991 and helped to formulate the Aβ hypothesis as one of the very important features of AD etiopathogenesis, progress in AD treatment utilizing BACE1 inhibitors has remained limited. Moreover, in the last years, major pharmaceutical companies have discontinued clinical trials of five BACE1 inhibitors that had been strongly perceived as prospective. In our review, the Aβ hypothesis, the enzyme, its functions, and selected substrates are described. BACE1 inhibitors are classified into four generations. Those that underwent clinical trials displayed adverse effects, including weight loss, skin rashes, worsening of neuropsychiatric symptoms, etc. Some inhibitors could not establish a statistically significant risk-benefit ratio, or even scored worse than placebo. We still believe that drugs targeting BACE1 may still hide some potential, but a different approach to BACE1 inhibition or a shift of focus to modulation of its trafficking and/or post-translational modification should now be followed.

miR-149-5p inhibition reduces Alzheimer's disease β-amyloid generation in 293/APPsw cells by upregulating H4K16ac via KAT8

Alzheimer's disease (AD), the leading cause of age-related dementia, is characterized by abnormal β-amyloid accumulation. During learning, memory formation and consolidation, increased levels of histone H3 and H4 acetylation are observed. The present study reported significantly decreased level of H4K16ac in the plasma of patients with AD compared with healthy subjects via western blotting and reverse transcription-quantitative (RT-q)PCR. Lysine acetyltransferase 8 (KAT8) expression, the major lysine acetyltransferase responsible for the acetylation of H4K16, was significantly decreased in patients with AD compared with healthy subjects as determined via western blotting and RT-qPCR. The results indicated that aberrant expression patterns of H4K16ac and KAT8 might be associated with AD progression. Moreover, western blot analysis demonstrated that KAT8-overexpression cells displayed increased levels of H4K16ac, accompanied by higher levels of neuroprotective soluble amyloid precursor protein (sAPP)α and β-secretase (BACE)2, and decreased levels of sAPPβ and BACE1 compared with negative control and vector cells. In neurodegenerative disorders, microRNAs (miRNAs/miRs) are deregulated; however, the effect of miRNA dysregulation on histone acetylation is not completely understood. To the best of our knowledge, the present study identified a novel inhibitory interaction between miR-149-5p and KAT8 3'-UTR that contributed to the pathological alterations in an AD cell model for the first time, using bioinformatics and a dual-luciferase reporter assay. The western blotting results indicated that, compared with the inhibitor control group, miR-149-5p inhibitor markedly increased H4K16ac levels, which were significantly suppressed by co-transfection with KAT8 short hairpin (sh)RNA. KAT8 shRNA and miR-149-5p inhibitor co-transfection abolished the beneficial effects of miR-149-5p inhibitor. The results indicated that miR-149-5p regulated KAT8 and H4K16ac expression in an AD cell model, which may be associated with the pathological process of AD; therefore, miRNA may serve as a potential drug target for AD.

Identification of truncated C-terminal fragments of the Alzheimer's disease amyloid protein precursor derived from sequential proteolytic pathways

Recent evidence supports the emerging hypothesis that the amyloid-β precursor protein C-terminal fragments (APP-CTFs) and dysregulations in both their qualitative and quantitative productions may actively and directly contribute to the neuronal toxicity in early phases of Alzheimer's disease (AD). These new findings revealed the urgent needs and gaps in better understanding the metabolism and full spectrum of APP-CTFs. In this study, we characterized by mass spectrometry the full patterns of APP-CTFs in different cell types and in the brain of an AD APPPS1 mouse model. In these systems, we first discovered a series of 71-80 amino acids long N-terminally truncated APP-CTFs of unknown functions. We next demonstrated that these N-terminally truncated APP-CTFs are sequentially produced by the proteolytic processing of APP-C80, by an as yet unidentified protease. Finally, these N-terminally truncated APP-CTFs are likely protein substrates recognized and processed by the γ-secretase complex, leading to the production of N-terminally truncated Aβ peptides. Together, our findings provide new insights into the metabolism of APP and offer potential new strategies to modulate the production of toxic Aβ peptides in AD.

RCAN1 Inhibits BACE2 Turnover by Attenuating Proteasome-Mediated BACE2 Degradation

Amyloid-β protein (Aβ) is the main component of neuritic plaques, the pathological hallmark of Alzheimer's disease (AD). β-site APP cleaving enzyme 1 (BACE1) is a major β-secretase contributing to Aβ generation. β-site APP cleaving enzyme 2 (BACE2), the homolog of BACE1, is not only a θ-secretase but also a conditional β-secretase. Previous studies showed that regulator of calcineurin 1 (RCAN1) is markedly increased by AD and promotes BACE1 expression. However, the role of RCAN1 in BACE2 regulation remains elusive. Here, we showed that RCAN1 increases BACE2 protein levels. Moreover, RCAN1 inhibits the turnover of BACE2 protein. Furthermore, RCAN1 attenuates proteasome-mediated BACE2 degradation, but not lysosome-mediated BACE2 degradation. Taken together, our work indicates that RCAN1 inhibits BACE2 turnover by attenuating proteasome-mediated BACE2 degradation. It advances our understanding of BACE2 regulation and provides a potential mechanism of BACE2 dysregulation in AD.

BACE2 degradation is mediated by both the proteasome and lysosome pathways

BACKGROUND

Alzheimer's disease is the most common neurodegenerative disease in the elderly. Amyloid-β protein (Aβ) is the major component of neuritic plaques which are the hallmark of AD pathology. β-site APP cleaving enzyme 1 (BACE1) is the major β-secretase contributing to Aβ generation. β-site APP-cleaving enzyme 2 (BACE2), the homolog of BACE1, might play a complex role in the pathogenesis of Alzheimer's disease as it is not only a θ-secretase but also a conditional β-secretase. Dysregulation of BACE2 is observed in Alzheimer's disease. However, the regulation of BACE2 is less studied compared with BACE1, including its degradation pathways. In this study, we investigated the turnover rates and degradation pathways of BACE2 in both neuronal cells and non-neuronal cells.

RESULTS

Both lysosomal inhibition and proteasomal inhibition cause a time- and dose-dependent increase of transiently overexpressed BACE2 in HEK293 cells. The half-life of transiently overexpressed BACE2 protein is approximately 6 h. Moreover, the half-life of endogenous BACE2 protein is approximately 4 h in both HEK293 cells and mouse primary cortical neurons. Furthermore, both lysosomal inhibition and proteasomal inhibition markedly increases endogenous BACE2 in HEK293 cells and mouse primary cortical neurons.

CONCLUSIONS

This study demonstrates that BACE2 is degraded by both the proteasome and lysosome pathways in both neuronal and non-neuronal cells at endogenous level and in transient overexpression system. It indicates that BACE2 dysregulation might be mediated by the proteasomal and lysosomal impairment in Alzheimer's disease. This study advances our understanding of the regulation of BACE2 and provides a potential mechanism of its dysregulation in Alzheimer's disease.

A presenilin-1 mutation causes Alzheimer disease without affecting Notch signaling

Presenilin-1 (PSEN1) is the catalytic subunit of the γ-secretase complex, and pathogenic mutations in the PSEN1 gene account for the majority cases of familial AD (FAD). FAD-associated mutant PSEN1 proteins have been shown to affect APP processing and Aβ generation and inhibit Notch1 cleavage and Notch signaling. In this report, we found that a PSEN1 mutation (S169del) altered APP processing and Aβ generation, and promoted neuritic plaque formation as well as learning and memory deficits in AD model mice. However, this mutation did not affect Notch1 cleavage and Notch signaling in vitro and in vivo. Taken together, we demonstrated that PSEN1_S169del has distinct effects on APP processing and Notch1 cleavage, suggesting that Notch signaling may not be critical for AD pathogenesis and serine169 could be a critical site as a potential target for the development of novel γ-secretase modulators without affecting Notch1 cleavage to treat AD.

Transcriptional activation of USP16 gene expression by NFκB signaling

Ubiquitin Specific Peptidase 16 (USP16) has been reported to contribute to somatic stem-cell defects in Down syndrome. However, how this gene being regulated is largely unknown. To study the mechanism underlying USP16 gene expression, USP16 gene promoter was cloned and analyzed by luciferase assay. We identified that the 5′ flanking region (− 1856 bp ~ + 468 bp) of the human USP16 gene contained the functional promotor to control its transcription. Three bona fide NFκB binding sites were found in USP16 promoter. We showed that p65 overexpression enhanced endogenous USP16 mRNA level. Furthermore, LPS and TNFα, strong activators of the NFκB pathway, upregulated the USP16 transcription. Our data demonstrate that USP16 gene expression is tightly regulated at transcription level. NFκB signaling regulates the human USP16 gene expression through three cis-acting elements. The results provide novel insights into a potential role of dysregulation of USP16 expression in Alzheimer’s dementia in Down Syndrome.

Common BACE2 Polymorphisms are Associated with Altered Risk for Alzheimer's Disease and CSF Amyloid Biomarkers in APOE ε4 Non-Carriers

It was recently suggested that beta-site amyloid precursor protein (APP)-cleaving enzyme 2 (BACE2) functions as an amyloid beta (Aβ)-degrading enzyme; in addition to its better understood role as an APP secretase. Due to this finding we sought to understand the possible genetic risk contributed by the BACE2 locus to the development of late-onset Alzheimer’s disease (AD). In this study, we report that common single nucleotide polymorphism (SNP) variation in BACE2 is associated with altered AD risk in apolipoprotein E gene (APOE) epsilon 4 variant (ε4) non-carriers. In addition, in ε4 non-carriers diagnosed with AD or mild cognitive impairment (MCI), SNPs within the BACE2 locus are associated with cerebrospinal fluid (CSF) levels of Aβ1-42. Further, SNP variants in BACE2 are also associated with BACE2 RNA expression levels suggesting a potential mechanism for the CSF Aβ1-42 findings. Lastly, overexpression of BACE2 in vitro resulted in decreased Aβ1-40 and Aβ1-42 fragments in a cell line model of Aβ production. These findings suggest that genetic variation at the BACE2 locus modifies AD risk for those individuals who don’t carry the ε4 variant of APOE. Further, our data indicate that the biological mechanism associated with this altered risk is linked to amyloid generation or clearance possibly through BACE2 expression changes.

The role of membrane trafficking in the processing of amyloid precursor protein and production of amyloid peptides in Alzheimer's disease

Alzheimer's disease (AD) is characterized by progressive accumulation of misfolded proteins, which form senile plaques and neurofibrillary tangles, and the release of inflammatory mediators by innate immune responses. β-Amyloid peptide (Aβ) is derived from sequential processing of the amyloid precursor protein (APP) by membrane-bound proteases, namely the β-secretase, BACE1, and γ-secretase. Membrane trafficking plays a key role in the regulation of APP processing as both APP and the processing secretases traffic along distinct pathways. Genome wide sequencing studies have identified several AD susceptibility genes which regulate membrane trafficking events. To understand the pathogenesis of AD it is critical that the cell biology of APP and Aβ production in neurons is well defined. This review discusses recent advances in unravelling the membrane trafficking events associated with the production of Aβ, and how AD susceptible alleles may perturb the sorting and transport of APP and BACE1. Mechanisms whereby inflammation may influence APP processing are also considered.

BACE2, a conditional β-secretase, contributes to Alzheimer's disease pathogenesis

Deposition of amyloid-β protein (Aβ) to form neuritic plaques is the characteristic neuropathology of Alzheimer's disease (AD). Aβ is generated from amyloid precursor protein (APP) by β- and γ-secretase cleavages. BACE1 is the β-secretase and its inhibition induces severe side effects, whereas its homolog BACE2 normally suppresses Aβ by cleaving APP/Aβ at the θ-site (Phe20) within the Aβ domain. Here, we report that BACE2 also processes APP at the β site, and the juxtamembrane helix (JH) of APP inhibits its β-secretase activity, enabling BACE2 to cleave nascent APP and aggravate AD symptoms. JH-disrupting mutations and clusterin binding to JH triggered BACE2-mediated β-cleavage. Both BACE2 and clusterin were elevated in aged mouse brains, and enhanced β-cleavage during aging. Therefore, BACE2 contributes to AD pathogenesis as a conditional β-secretase and could be a preventive and therapeutic target for AD without the side effects of BACE1 inhibition.

Identification of Genes Associated With Hirschsprung Disease, Based on Whole-Genome Sequence Analysis, and Potential Effects on Enteric Nervous System Development

BACKGROUND & AIMS

Hirschsprung disease, or congenital aganglionosis, is believed to be oligogenic-that is, caused by multiple genetic factors. We performed whole-genome sequence analyses of patients with Hirschsprung disease to identify genetic factors that contribute to disease development and analyzed the functional effects of these variants.

METHODS

We performed whole-genome sequence analyses of 443 patients with short-segment disease, recruited from hospitals in China and Vietnam, and 493 ethnically matched individuals without Hirschsprung disease (controls). We performed genome-wide association analyses and gene-based rare-variant burden tests to identify rare and common disease-associated variants and study their interactions. We obtained induced pluripotent stem cell (iPSC) lines from 4 patients with Hirschsprung disease and 2 control individuals, and we used these to generate enteric neural crest cells for transcriptomic analyses. We assessed the neuronal lineage differentiation capability of iPSC-derived enteric neural crest cells using an in vitro differentiation assay.

RESULTS

We identified 4 susceptibility loci, including 1 in the phospholipase D1 gene (PLD1) (P = 7.4 × 10^-7). The patients had a significant excess of rare protein-altering variants in genes previously associated with Hirschsprung disease and in the β-secretase 2 gene (BACE2) (P = 2.9 × 10^-6). The epistatic effects of common and rare variants across these loci provided a sensitized background that increased risk for the disease. In studies of the iPSCs, we observed common and distinct pathways associated with variants in RET that affect risk. In functional assays, we found variants in BACE2 to protect enteric neurons from apoptosis. We propose that alterations in BACE1 signaling via amyloid β precursor protein and BACE2 contribute to pathogenesis of Hirschsprung disease.

CONCLUSIONS

In whole-genome sequence analyses of patients with Hirschsprung disease, we identified rare and common variants associated with disease risk. Using iPSC cells, we discovered some functional effects of these variants.

Trisomy of human chromosome 21 enhances amyloid-β deposition independently of an extra copy of APP

Down syndrome, caused by trisomy of chromosome 21, is the single most common risk factor for early-onset Alzheimer's disease. Worldwide approximately 6 million people have Down syndrome, and all these individuals will develop the hallmark amyloid plaques and neurofibrillary tangles of Alzheimer's disease by the age of 40 and the vast majority will go on to develop dementia. Triplication of APP, a gene on chromosome 21, is sufficient to cause early-onset Alzheimer's disease in the absence of Down syndrome. However, whether triplication of other chromosome 21 genes influences disease pathogenesis in the context of Down syndrome is unclear. Here we show, in a mouse model, that triplication of chromosome 21 genes other than APP increases amyloid-β aggregation, deposition of amyloid-β plaques and worsens associated cognitive deficits. This indicates that triplication of chromosome 21 genes other than APP is likely to have an important role to play in Alzheimer's disease pathogenesis in individuals who have Down syndrome. We go on to show that the effect of trisomy of chromosome 21 on amyloid-β aggregation correlates with an unexpected shift in soluble amyloid-β 40/42 ratio. This alteration in amyloid-β isoform ratio occurs independently of a change in the carboxypeptidase activity of the γ-secretase complex, which cleaves the peptide from APP, or the rate of extracellular clearance of amyloid-β. These new mechanistic insights into the role of triplication of genes on chromosome 21, other than APP, in the development of Alzheimer's disease in individuals who have Down syndrome may have implications for the treatment of this common cause of neurodegeneration.

Cleavage of potassium channel Kv2.1 by BACE2 reduces neuronal apoptosis

Potassium channel Kv2.1 regulates potassium current in cortical neurons and potassium efflux is necessary for cell apoptosis. As a major component of delayed rectifier current potassium channels, Kv2.1 forms clusters in the membrane of hippocampal neurons. BACE2 is an aspartyl protease to cleave APP to prevent the generation of Aβ, a central component of neuritic plaques in Alzheimer's brain. We now identified Kv2.1 as a novel substrate of BACE2. We found that BACE2 cleaved Kv2.1 at Thr376, Ala717, and Ser769 sites and disrupted Kv2.1 clustering on cell membrane, resulting in decreased I_k of Kv2.1 and a hyperpolarizing shift in primary neurons. Furthermore, we discovered that the BACE2-cleaved Kv2.1 forms, Kv2.1-1-375, Kv2.1-1-716, and Kv2.1-1-768, depressed the delayed rectifier I_k surge and reduced neuronal apoptosis. Our study suggests that BACE2 plays a neuroprotective role by cleavage of Kv2.1 to prevent the outward potassium currents, a potential new target for Alzheimer's treatment.

The Impact of APP on Alzheimer-like Pathogenesis and Gene Expression in Down Syndrome iPSC-Derived Neurons

Early-onset Alzheimer disease (AD)-like pathology in Down syndrome is commonly attributed to an increased dosage of the amyloid precursor protein (APP) gene. To test this in an isogenic human model, we deleted the supernumerary copy of the APP gene in trisomic Down syndrome induced pluripotent stem cells or upregulated APP expression in euploid human pluripotent stem cells using CRISPRa. Cortical neuronal differentiation shows that an increased APP gene dosage is responsible for increased β-amyloid production, altered Aβ42/40 ratio, and deposition of the pyroglutamate (E3)-containing amyloid aggregates, but not for several tau-related AD phenotypes or increased apoptosis. Transcriptome comparisons demonstrate that APP has a widespread and temporally modulated impact on neuronal gene expression. Collectively, these data reveal an important role for APP in the amyloidogenic aspects of AD but challenge the idea that increased APP levels are solely responsible for increasing specific phosphorylated forms of tau or enhanced neuronal cell death in Down syndrome-associated AD pathogenesis.

Outstanding Phenotypic Differences in the Profile of Amyloid-β between Tg2576 and APPswe/PS1dE9 Transgenic Mouse Models of Alzheimer's Disease

APPswe/PS1dE9 and Tg2576 are very common transgenic mouse models of Alzheimer’s disease (AD), used in many laboratories as tools to research the mechanistic process leading to the disease. In order to augment our knowledge about the amyloid-β (Aβ) isoforms present in both transgenic mouse models, we have developed two chromatographic methods, one acidic and the other basic, for the characterization of the Aβ species produced in the brains of the two transgenic mouse models. After immunoprecipitation and micro-liquid chromatography-electrospray ionization mass spectrometry/mass spectrometry, 10 species of Aβ, surprisingly all of human origin, were detected in the brain of Tg2576 mouse, whereas 39 species, of both murine and human origin, were detected in the brain of the APP/PS1 mouse. To the best of our knowledge, this is the first study showing the identification of such a high number of Aβ species in the brain of the APP/PS1 transgenic mouse, whereas, in contrast, a much lower number of Aβ species were identified in the Tg2576 mouse. Therefore, this study brings to light a relevant phenotypic difference between these two popular mice models of AD.

The type II transmembrane serine protease matriptase cleaves the amyloid precursor protein and reduces its processing to β-amyloid peptide

Recent studies have reported that many proteases, besides the canonical α-, β-, and γ-secretases, cleave the amyloid precursor protein (APP) and modulate β-amyloid (Aβ) peptide production. Moreover, specific APP isoforms contain Kunitz protease-inhibitory domains, which regulate the proteolytic activity of serine proteases. This prompted us to investigate the role of matriptase, a member of the type II transmembrane serine protease family, in APP processing. Using quantitative RT-PCR, we detected matriptase mRNA in several regions of the human brain with an enrichment in neurons. RNA sequencing data of human dorsolateral prefrontal cortex revealed relatively high levels of matriptase RNA in young individuals, whereas lower levels were detected in older individuals. We further demonstrate that matriptase and APP directly interact with each other and that matriptase cleaves APP at a specific arginine residue (Arg-102) both in vitro and in cells. Site-directed (Arg-to-Ala) mutagenesis of this cleavage site abolished matriptase-mediated APP processing. Moreover, we observed that a soluble, shed matriptase form cleaves endogenous APP in SH-SY5Y cells and that this cleavage significantly reduces APP processing to Aβ40. In summary, this study identifies matriptase as an APP-cleaving enzyme, an activity that could have important consequences for the abundance of Aβ and in Alzheimer's disease pathology.

Modifications and Trafficking of APP in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD), the most common neurodegenerative disorder, is the leading cause of dementia. Neuritic plaque, one of the major characteristics of AD neuropathology, mainly consists of amyloid β (Aβ) protein. Aβ is derived from amyloid precursor protein (APP) by sequential cleavages of β- and γ-secretase. Although APP upregulation can promote AD pathogenesis by facilitating Aβ production, growing evidence indicates that aberrant post-translational modifications and trafficking of APP play a pivotal role in AD pathogenesis by dysregulating APP processing and Aβ generation. In this report, we reviewed the current knowledge of APP modifications and trafficking as well as their role in APP processing. More importantly, we discussed the effect of aberrant APP modifications and trafficking on Aβ generation and the underlying mechanisms, which may provide novel strategies for drug development in AD.