Amyloid precursor protein and endosomal-lysosomal dysfunction in Alzheimer's disease: inseparable partners in a multifactorial disease

Mutation of TRPML1 Channel and Pathogenesis of Neurodegeneration in Haimeria

Neurodegenerative diseases, a group of debilitating disorders, have garnered increasing attention due to their escalating prevalence, particularly among aging populations. Alzheimer's disease (AD) reigns as a prominent exemplar within this category, distinguished by its relentless progression of cognitive impairment and the accumulation of aberrant protein aggregates within the intricate landscape of the brain. While the intricate pathogenesis of neurodegenerative diseases has been the subject of extensive investigation, recent scientific inquiry has unveiled a novel player in this complex scenario-transient receptor potential mucolipin 1 (TRPML1) channels. This comprehensive review embarks on an exploration of the intricate interplay between TRPML1 channels and neurodegenerative diseases, with an explicit spotlight on Alzheimer's disease. It immerses itself in the intricate molecular mechanisms governing TRPML1 channel functionality and elucidates their profound implications for the well-being of neurons. Furthermore, the review ventures into the realm of therapeutic potential, pondering the possibilities and challenges associated with targeting TRPML1 channels as a promising avenue for the amelioration of neurodegenerative disorders. As we traverse this multifaceted terrain of neurodegeneration and the enigmatic role of TRPML1 channels, we embark on a journey that not only broadens our understanding of the intricate machinery governing neuronal health but also holds promise for the development of innovative therapeutic interventions in the relentless battle against neurodegenerative diseases.

γ-Secretase Modulator BPN15606 Reduced Aβ42 and Aβ40 and Countered Alzheimer-Related Pathologies in a Mouse Model of Down Syndrome

OBJECTIVES

Due to increased gene dose for the amyloid precursor protein (APP), elderly adults with Down syndrome (DS) are at a markedly increased risk of Alzheimer's disease (AD), known as DS-AD. How the increased APP gene dose acts and which APP products are responsible for DS-AD is not well understood, thus limiting strategies to target pathogenesis. As one approach to address this question, we used a novel class of γ-secretase modulators that promote γ-site cleavages by the γ-secretase complex, resulting in lower levels of the Aβ42 and Aβ40 peptides.

METHODS

Ts65Dn mice, which serve as a model of DS, were treated via oral gavage with 10 mg/kg/weekday of BPN15606 (a potent and novel pyridazine-containing γ-secretase modulators). Treatment started at 3 months-of-age and lasted for 4 months.

RESULTS

Demonstrating successful target engagement, treatment with BPN15606 significantly decreased levels of Aβ40 and Aβ42 in the cortex and hippocampus; it had no effect on full-length APP or its C-terminal fragments in either 2 N or Ts65Dn mice. Importantly, the levels of total amyloid-β were not impacted, pointing to BPN15606-mediated enhancement of processivity of γ-secretase. Additionally, BPN15606 rescued hyperactivation of Rab5, a protein responsible for regulating endosome function, and normalized neurotrophin signaling deficits. BPN15606 treatment also normalized the levels of synaptic proteins and tau phosphorylation, while reducing astrocytosis and microgliosis, and countering cognitive deficits.

INTERPRETATION

Our findings point to the involvement of increased levels of Aβ42 and/or Aβ40 in contributing to several molecular and cognitive traits associated with DS-AD. They speak to increased dosage of the APP gene acting through heightened levels of Aβ42 and/or Aβ40 as supporting pathogenesis. These findings further the interest in the potential use of γ-secretase modulators for treating and possibly preventing AD in individuals with DS. ANN NEUROL 2024;96:390-404.

Reduction of neuroinflammation and seizures in a mouse model of CLN1 batten disease using the small molecule enzyme mimetic, N-Tert-butyl hydroxylamine

Infantile neuronal ceroid lipofuscinosis (CLN1 Batten Disease) is a devastating pediatric lysosomal storage disease caused by pathogenic variants in the CLN1 gene, which encodes the depalmitoylation enzyme, palmitoyl-protein thioesterase 1 (PPT1). CLN1 patients present with visual deterioration, psychomotor dysfunction, and recurrent seizures until neurodegeneration results in death, typically before fifteen years of age. Histopathological features of CLN1 include aggregation of lysosomal autofluorescent storage material (AFSM), as well as profound gliosis. The current management of CLN1 is relegated to palliative care. Here, we examine the therapeutic potential of a small molecule PPT1 mimetic, N-tert-butyl hydroxylamine (NtBuHA), in a Cln1-/- mouse model. Treatment with NtBuHA reduced AFSM accumulation both in vitro and in vivo. Importantly, NtBuHA treatment in Cln1-/- mice reduced neuroinflammation, mitigated epileptic episodes, and normalized motor function. Live cell imaging of Cln1-/- primary cortical neurons treated with NtBuHA partially rescued aberrant synaptic calcium dynamics, suggesting a potential mechanism contributing to the therapeutic effects of NtBuHA in vivo. Taken together, our findings provide supporting evidence for NtBuHA as a potential treatment for CLN1 Batten Disease.

Truncated tau interferes with the autophagy and endolysosomal pathway and results in lipid accumulation

The autophagy-lysosomal pathway plays a critical role in the clearance of tau protein aggregates that deposit in the brain in tauopathies, and defects in this system are associated with disease pathogenesis. Here, we report that expression of Tau35, a tauopathy-associated carboxy-terminal fragment of tau, leads to lipid accumulation in cell lines and primary cortical neurons. Our findings suggest that this is likely due to a deleterious block of autophagic clearance and lysosomal degradative capacity by Tau35. Notably, upon induction of autophagy by Torin 1, Tau35 inhibited nuclear translocation of transcription factor EB (TFEB), a key regulator of lysosomal biogenesis. Both cell lines and primary cortical neurons expressing Tau35 also exhibited changes in endosomal protein expression. These findings implicate autophagic and endolysosomal dysfunction as key pathological mechanisms through which disease-associated tau fragments could lead to the development and progression of tauopathy.

A systems biology-based identification and in vivo functional screening of Alzheimer's disease risk genes reveal modulators of memory function

Genome-wide association studies (GWASs) have uncovered over 75 genomic loci associated with risk for late-onset Alzheimer's disease (LOAD), but identification of the underlying causal genes remains challenging. Studies of induced pluripotent stem cell (iPSC)-derived neurons from LOAD patients have demonstrated the existence of neuronal cell-intrinsic functional defects. Here, we searched for genetic contributions to neuronal dysfunction in LOAD using an integrative systems approach that incorporated multi-evidence-based gene mapping and network-analysis-based prioritization. A systematic perturbation screening of candidate risk genes in Caenorhabditis elegans (C. elegans) revealed that neuronal knockdown of the LOAD risk gene orthologs vha-10 (ATP6V1G2), cmd-1 (CALM3), amph-1 (BIN1), ephx-1 (NGEF), and pho-5 (ACP2) alters short-/intermediate-term memory function, the cognitive domain affected earliest during LOAD progression. These results highlight the impact of LOAD risk genes on evolutionarily conserved memory function, as mediated through neuronal endosomal dysfunction, and identify new targets for further mechanistic interrogation.

Increasing the Survival of a Neuronal Model of Alzheimer's Disease Using Docosahexaenoic Acid, Restoring Endolysosomal Functioning by Modifying the Interactions between the Membrane Proteins C99 and Rab5

Docosahexaenoic acid (DHA, C22:6 ω3) may be involved in various neuroprotective mechanisms that could prevent Alzheimer's disease (AD). Its influence has still been little explored regarding the dysfunction of the endolysosomal pathway, known as an early key event in the physiopathological continuum triggering AD. This dysfunction could result from the accumulation of degradation products of the precursor protein of AD, in particular the C99 fragment, capable of interacting with endosomal proteins and thus contributing to altering this pathway from the early stages of AD. This study aims to evaluate whether neuroprotection mediated by DHA can also preserve the endolysosomal function. AD-typical endolysosomal abnormalities were recorded in differentiated human SH-SY5Y neuroblastoma cells expressing the Swedish form of human amyloid precursor protein. This altered phenotype included endosome enlargement, the reduced secretion of exosomes, and a higher level of apoptosis, which confirmed the relevance of the cellular model chosen for studying the associated deleterious mechanisms. Second, neuroprotection mediated by DHA was associated with a reduced interaction of C99 with the Rab5 GTPase, lower endosome size, restored exosome production, and reduced neuronal apoptosis. Our data reveal that DHA may influence protein localization and interactions in the neuronal membrane environment, thereby correcting the dysfunction of endocytosis and vesicular trafficking associated with AD.

Engineered Antibodies to Improve Efficacy against Neurodegenerative Disorders

Antibodies that can selectively remove rogue proteins in the brain are an obvious choice to treat neurodegenerative disorders (NDs), but after decades of efforts, only two antibodies to treat Alzheimer's disease are approved, dozens are in the testing phase, and one was withdrawn, and the other halted, likely due to efficacy issues. However, these outcomes should have been evident since these antibodies cannot enter the brain sufficiently due to the blood-brain barrier (BBB) protectant. However, all products can be rejuvenated by binding them with transferrin, preferably as smaller fragments. This model can be tested quickly and at a low cost and should be applied to bapineuzumab, solanezumab, crenezumab, gantenerumab, aducanumab, lecanemab, donanemab, cinpanemab, and gantenerumab, and their fragments. This paper demonstrates that conjugating with transferrin does not alter the binding to brain proteins such as amyloid-β (Aβ) and α-synuclein. We also present a selection of conjugate designs that will allow cleavage upon entering the brain to prevent their exocytosis while keeping the fragments connected to enable optimal binding to proteins. The identified products can be readily tested and returned to patients with the lowest regulatory cost and delays. These engineered antibodies can be manufactured by recombinant engineering, preferably by mRNA technology, as a more affordable solution to meet the dire need to treat neurodegenerative disorders effectively.

A clathrin mediated endocytosis scaffolding protein, Intersectin 1, changes in an isoform, brain region, and sex specific manner in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia and is characterized by the accumulation of amyloid-beta (Aβ) plaques and neurofibrillary Tau tangles in the brain. We previously identified a set of candidate AD microRNAs (miRNAs) in human cerebrospinal fluid (CSF) and used a target prediction pipeline to identify mRNAs and pathways that could potentially be regulated by the miRNAs. Of these pathways, clathrin mediated endocytosis (CME) was selected for further investigation. CME is altered in multiple brain cell types in AD and is implicated in early cellular phenotypes such as enlarged early endosomes and pathogenic processing of Aβ. However, a comprehensive evaluation of major CME hub proteins in humans with AD across multiple brain regions is lacking. Thus, we used immunoblots to evaluate human post-mortem AD and control (CTL) frontal cortex (FC; AD n = 22, CTL n = 23) and hippocampus (HP; AD n = 34, CTL n = 22) for changes in Intersectin 1 (ITSN1), Phosphatidylinositol Binding Clathrin Assembly Protein gene (PICALM), Clathrin Light Chain (CLT), FCH and Mu Domain Containing Endocytic Adaptor 1 (FCHO1), Adaptor Related Protein Complex 2 (AP2) Subunit Alpha 1 (AP2A1), and Dynamin 2 (DNM2). Of these, we found that in AD, ITSN1-long (ITSN1-L) was decreased in the FC of males and HP of females, while ITSN1-short was increased in the HP of both males and females. We further evaluated ITSN1-L levels in cortex (CTX) and HP of the 5xFAD mouse model of Aβ pathology at different timepoints during aging and disease progression by immunoblot (n = 5-8 per group). At 3 months, female 5xFAD exhibited an increase of ITSN1-L in CTX but a decrease at 6 and 9 months. Additionally, immunofluorescent staining of 5xFAD primary HP neurons showed an increase of ITSN1-L in matured 5xFAD neurons at 21 and 28 days in vitro. Together, our studies show that in AD, isoforms of ITSN1 change in a brain region-and sex-dependent manner. Further, changes in ITSN1-L are transient with levels increasing during early Aβ accumulation and decreasing during later progression. These findings suggest that ITSN1 expression, and consequently CME activity, may change depending on the stage of disease progression.

Long term rescue of Alzheimer's deficits in vivo by one-time gene-editing of App C-terminus

Gene-editing technologies promise to create a new class of therapeutics that can achieve permanent correction with a single intervention. Besides eliminating mutant alleles in familial disease, gene-editing can also be used to favorably manipulate upstream pathophysiologic events and alter disease-course in wider patient populations, but few such feasible therapeutic avenues have been reported. Here we use CRISPR-Cas9 to edit the last exon of amyloid precursor protein (App), relevant for Alzheimer's disease (AD). Our strategy effectively eliminates an endocytic (YENPTY) motif at APP C-terminus, while preserving the N-terminus and compensatory APP-homologues. This manipulation favorably alters events along the amyloid-pathway - inhibiting toxic APP-β-cleavage fragments (including Aβ) and upregulating neuroprotective APP-α-cleavage products. AAV-driven editing ameliorates neuropathologic, electrophysiologic, and behavioral deficits in an AD knockin mouse model. Effects persist for many months, and no abnormalities are seen in WT mice even after germline App-editing; underlining overall efficacy and safety. Pathologic alterations in the glial-transcriptome of App-KI mice, as seen by single nuclei RNA-sequencing (sNuc-Seq), are also normalized by App C-terminus editing. Our strategy takes advantage of innate transcriptional rules that render terminal exons insensitive to nonsense-decay, and the upstream manipulation is expected to be effective for all forms of AD. These studies offer a path for a one-time disease-modifying treatment for AD.

Correlative cryo-soft X-ray tomography and cryo-structured illumination microscopy reveal changes to lysosomes in amyloid-β-treated neurons

Protein misfolding is common to neurodegenerative diseases (NDs) including Alzheimer's disease (AD), which is partly characterized by the self-assembly and accumulation of amyloid-beta in the brain. Lysosomes are a critical component of the proteostasis network required to degrade and recycle material from outside and within the cell and impaired proteostatic mechanisms have been implicated in NDs. We have previously established that toxic amyloid-beta oligomers are endocytosed, accumulate in lysosomes, and disrupt the endo-lysosomal system in neurons. Here, we use pioneering correlative cryo-structured illumination microscopy and cryo-soft X-ray tomography imaging techniques to reconstruct 3D cellular architecture in the native state revealing reduced X-ray density in lysosomes and increased carbon dense vesicles in oligomer treated neurons compared with untreated cells. This work provides unprecedented visual information on the changes to neuronal lysosomes inflicted by amyloid beta oligomers using advanced methods in structural cell biology.

Apolipoprotein E2 Expression Alters Endosomal Pathways in a Mouse Model With Increased Brain Exosome Levels During Aging

The polymorphic APOE gene is the greatest genetic determinant of sporadic Alzheimer's disease risk: the APOE4 allele increases risk, while the APOE2 allele is neuroprotective compared with the risk-neutral APOE3 allele. The neuronal endosomal system is inherently vulnerable during aging, and APOE4 exacerbates this vulnerability by driving an enlargement of early endosomes and reducing exosome release in the brain of humans and mice. We hypothesized that the protective effects of APOE2 are, in part, mediated through the endosomal pathway. Messenger RNA analyses showed that APOE2 leads to an enrichment of endosomal pathways in the brain when compared with both APOE3 and APOE4. Moreover, we show age-dependent alterations in the recruitment of key endosomal regulatory proteins to vesicle compartments when comparing APOE2 to APOE3. In contrast to the early endosome enlargement previously shown in Alzheimer's disease and APOE4 models, we detected similar morphology and abundance of early endosomes and retromer-associated vesicles within cortical neurons of aged APOE2 targeted-replacement mice compared with APOE3. Additionally, we observed increased brain extracellular levels of endosome-derived exosomes in APOE2 compared with APOE3 mice during aging, consistent with enhanced endosomal cargo clearance by exosomes to the extracellular space. Our findings thus demonstrate that APOE2 enhances an endosomal clearance pathway, which has been shown to be impaired by APOE4 and which may be protective due to APOE2 expression during brain aging.

Clathrin mediated endocytosis in Alzheimer's disease: cell type specific involvement in amyloid beta pathology

This review provides a comprehensive examination of the role of clathrin-mediated endocytosis (CME) in Alzheimer's disease (AD) pathogenesis, emphasizing its impact across various cellular contexts beyond neuronal dysfunction. In neurons, dysregulated CME contributes to synaptic dysfunction, amyloid beta (Aβ) processing, and Tau pathology, highlighting its involvement in early AD pathogenesis. Furthermore, CME alterations extend to non-neuronal cell types, including astrocytes and microglia, which play crucial roles in Aβ clearance and neuroinflammation. Dysregulated CME in these cells underscores its broader implications in AD pathophysiology. Despite significant progress, further research is needed to elucidate the precise mechanisms underlying CME dysregulation in AD and its therapeutic implications. Overall, understanding the complex interplay between CME and AD across diverse cell types holds promise for identifying novel therapeutic targets and interventions.

Differential effects of SORL1 deficiency on the endo-lysosomal network in human neurons and microglia

The endosomal gene SORL1 is a strong Alzheimer's disease (AD) risk gene that harbours loss-of-function variants causative for developing AD. The SORL1 protein SORL1/SORLA is an endosomal receptor that interacts with the multi-protein sorting complex retromer to traffic various cargo through the endo-lysosomal network (ELN). Impairments in endo-lysosomal trafficking are an early cellular symptom in AD and a novel therapeutic target. However, the cell types of the central nervous system are diverse and use the ELN differently. If this pathway is to be effectively therapeutically targeted, understanding how key molecules in the ELN function in various cell types and how manipulating them affects cell-type specific responses relative to AD is essential. Here, we discuss an example where deficiency of SORL1 expression in a human model leads to stress on early endosomes and recycling endosomes in neurons, but preferentially leads to stress on lysosomes in microglia. The differences observed in these organelles could relate to the unique roles of these cells in the brain as neurons are professional secretory cells and microglia are professional phagocytic cells. Experiments to untangle these differences are fundamental to advancing the understanding of cell biology in AD and elucidating important pathways for therapeutic development. Human-induced pluripotent stem cell models are a valuable platform for such experiments. This article is part of a discussion meeting issue 'Understanding the endo-lysosomal network in neurodegeneration'.

From understanding to action: Exploring molecular connections of Down syndrome to Alzheimer's disease for targeted therapeutic approach

Down syndrome (DS) is caused by a third copy of chromosome 21. Alzheimer's disease (AD) is a neurodegenerative condition characterized by the deposition of amyloid-beta (Aβ) plaques and neurofibrillary tangles in the brain. Both disorders have elevated Aβ, tau, dysregulated immune response, and inflammation. In people with DS, Hsa21 genes like APP and DYRK1A are overexpressed, causing an accumulation of amyloid and neurofibrillary tangles, and potentially contributing to an increased risk of AD. As a result, people with DS are a key demographic for research into AD therapeutics and prevention. The molecular links between DS and AD shed insights into the underlying causes of both diseases and highlight potential therapeutic targets. Also, using biomarkers for early diagnosis and treatment monitoring is an active area of research, and genetic screening for high-risk individuals may enable earlier intervention. Finally, the fundamental mechanistic parallels between DS and AD emphasize the necessity for continued research into effective treatments and prevention measures for DS patients at risk for AD. Genetic screening with customized therapy approaches may help the DS population in current clinical studies and future biomarkers.

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.

Increased excitability of dentate gyrus mossy cells occurs early in life in the Tg2576 model of Alzheimer's disease

INTRODUCTION

Hyperexcitability in Alzheimer's disease (AD) emerge early and contribute to disease progression. The dentate gyrus (DG) is implicated in hyperexcitability in AD. We hypothesized that mossy cells (MCs), regulators of DG excitability, contribute to early hyperexcitability in AD. Indeed, MCs generate hyperexcitability in epilepsy.

METHODS

Using the Tg2576 model and WT mice (∼1month-old), we compared MCs electrophysiologically, assessed c-Fos activity marker, Aβ expression and mice performance in a hippocampal-dependent memory task.

RESULTS

Tg2576 MCs exhibit increased spontaneous excitatory events and decreased inhibitory currents, increasing the charge transfer excitation/inhibition ratio. Tg2576 MC intrinsic excitability was enhanced, and showed higher c-Fos, intracellular Aβ expression, and axon sprouting. Granule cells only showed changes in synaptic properties, without intrinsic changes. The effects occurred before a memory task is affected.

DISCUSSION

Early electrophysiological and morphological alterations in Tg2576 MCs are consistent with enhanced excitability, suggesting an early role in DG hyperexcitability and AD pathophysiology.

HIGHLIGHTS

∘ MCs from 1 month-old Tg2576 mice had increased spontaneous excitatory synaptic input. ∘ Tg2576 MCs had reduced spontaneous inhibitory synaptic input. ∘ Several intrinsic properties were abnormal in Tg2576 MCs. ∘ Tg2576 GCs had enhanced synaptic excitation but no changes in intrinsic properties. ∘ Tg2576 MCs exhibited high c-Fos expression, soluble Aβ and axonal sprouting.

Quantification of AMPA receptor subunits and RNA editing-related proteins in the J20 mouse model of Alzheimer's disease by capillary western blotting

Introduction

Accurate modelling of molecular changes in Alzheimer's disease (AD) dementia is crucial for understanding the mechanisms driving neuronal pathology and for developing treatments. Synaptic dysfunction has long been implicated as a mechanism underpinning memory dysfunction in AD and may result in part from changes in adenosine deaminase acting on RNA (ADAR) mediated RNA editing of the GluA2 subunit of AMPA receptors and changes in AMPA receptor function at the post synaptic cleft. However, few studies have investigated changes in proteins which influence RNA editing and notably, AD studies that focus on studying changes in protein expression, rather than changes in mRNA, often use traditional western blotting.

Methods

Here, we demonstrate the value of automated capillary western blotting to investigate the protein expression of AMPA receptor subunits (GluA1-4), the ADAR RNA editing proteins (ADAR1-3), and proteins known to regulate RNA editing (PIN1, WWP2, FXR1P, and CREB1), in the J20 AD mouse model. We describe extensive optimisation and validation of the automated capillary western blotting method, demonstrating the use of total protein to normalise protein load, in addition to characterising the optimal protein/antibody concentrations to ensure accurate protein quantification. Following this, we assessed changes in proteins of interest in the hippocampus of 44-week-old J20 AD mice.

Results

We observed an increase in the expression of ADAR1 p110 and GluA3 and a decrease in ADAR2 in the hippocampus of 44-week-old J20 mice. These changes signify a shift in the balance of proteins that play a critical role at the synapse. Regression analysis revealed unique J20-specific correlations between changes in AMPA receptor subunits, ADAR enzymes, and proteins that regulate ADAR stability in J20 mice, highlighting potential mechanisms mediating RNA-editing changes found in AD.

Discussion

Our findings in J20 mice generally reflect changes seen in the human AD brain. This study underlines the importance of novel techniques, like automated capillary western blotting, to assess protein expression in AD. It also provides further evidence to support the hypothesis that a dysregulation in RNA editing-related proteins may play a role in the initiation and/or progression of AD.

Nanotechnology for microglial targeting and inhibition of neuroinflammation underlying Alzheimer's pathology

BACKGROUND

Alzheimer's disease (AD) is considered to have a multifactorial etiology. The hallmark of AD is progressive neurodegeneration, which is characterized by the deepening loss of memory and a high mortality rate in the elderly. The neurodegeneration in AD is believed to be exacerbated following the intercoupled cascades of extracellular amyloid beta (Aβ) plaques, uncontrolled microglial activation, and neuroinflammation. Current therapies for AD are mostly designed to target the symptoms, with limited ability to address the mechanistic triggers for the disease. In this study, we report a novel nanotechnology based on microglial scavenger receptor (SR)-targeting amphiphilic nanoparticles (NPs) for the convergent alleviation of fibril Aβ (fAβ) burden, microglial modulation, and neuroprotection.

METHODS

We designed a nanotechnology approach to regulate the SR-mediated intracellular fAβ trafficking within microglia. We synthesized SR-targeting sugar-based amphiphilic macromolecules (AM) and used them as a bioactive shell to fabricate serum-stable AM-NPs via flash nanoprecipitation. Using electron microscopy, in vitro approaches, ELISA, and confocal microscopy, we investigated the effect of AM-NPs on Aβ fibrilization, fAβ-mediated microglial inflammation, and neurotoxicity in BV2 microglia and SH-SY5Y neuroblastoma cell lines.

RESULTS

AM-NPs interrupted Aβ fibrilization, attenuated fAβ microglial internalization via targeting the fAβ-specific SRs, arrested the fAβ-mediated microglial activation and pro-inflammatory response, and accelerated lysosomal degradation of intracellular fAβ. Moreover, AM-NPs counteracted the microglial-mediated neurotoxicity after exposure to fAβ.

CONCLUSIONS

The AM-NP nanotechnology presents a multifactorial strategy to target pathological Aβ aggregation and arrest the fAβ-mediated pathological progression in microglia and neurons.

The Contributions of the Endolysosomal Compartment and Autophagy to APOEɛ4 Allele-Mediated Increase in Alzheimer's Disease Risk

Apolipoprotein E4 (APOE4), although yet-to-be fully understood, increases the risk and lowers the age of onset of Alzheimer's disease (AD), which is the major cause of dementia among elderly individuals. The endosome-lysosome and autophagy pathways, which are necessary for homeostasis in both neurons and glia, are dysregulated even in early AD. Nonetheless, the contributory roles of these pathways to developing AD-related pathologies in APOE4 individuals and models are unclear. Therefore, this review summarizes the dysregulations in the endosome-lysosome and autophagy pathways in APOE4 individuals and non-human models, and how these anomalies contribute to developing AD-relevant pathologies. The available literature suggests that APOE4 causes endosomal enlargement, increases endosomal acidification, impairs endosomal recycling, and downregulates exosome production. APOE4 impairs autophagy initiation and inhibits basal autophagy and autophagy flux. APOE4 promotes lysosome formation and trafficking and causes ApoE to accumulate in lysosomes. APOE4-mediated changes in the endosome, autophagosome and lysosome could promote AD-related features including Aβ accumulation, tau hyperphosphorylation, glial dysfunction, lipid dyshomeostasis, and synaptic defects. ApoE4 protein could mediate APOE4-mediated endosome-lysosome-autophagy changes. ApoE4 impairs vesicle recycling and endosome trafficking, impairs the synthesis of autophagy genes, resists being dissociated from its receptors and degradation, and forms a stable folding intermediate that could disrupt lysosome structure. Drugs such as molecular correctors that target ApoE4 molecular structure and enhance autophagy may ameliorate the endosome-lysosome-autophagy-mediated increase in AD risk in APOE4 individuals.

Phosphorylation-state dependent intraneuronal sorting of Aβ differentially impairs autophagy and the endo-lysosomal system

ABBREVIATIONS

AD: Alzheimer disease; APP: amyloid beta precursor protein; ATG: autophagy related; Aβ: amyloid-β; CTSD: cathepsin D; DAPI: 4',6-diamidino-2-phenylindole; EEA1: early endosome antigen 1; FA: formic acid; GFP: green fluorescent protein; LAMP2: lysosomal-associated membrane protein 2; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MAP2: microtubule-associated protein 2; nmAβ: non-modified amyloid-β; npAβ: non-phosphorylated amyloid-β; pAβ: phosphorylated amyloid-β; p-Ser26Aβ: amyloid-β phosphorylated at serine residue 26; p-Ser8Aβ: amyloid-β phosphorylated at serine residue 8; RAB: RAB, member RAS oncogene family; RFP: red fluorescent protein; SQSTM1/p62: sequestome 1; YFP: yellow fluorescent protein.

Hippocampal TMEM55B overexpression in the 5XFAD mouse model of Alzheimer's disease

Dysfunction of the endosomal-lysosomal network is a notable feature of Alzheimer's disease (AD) pathology. Dysfunctional endo-lysosomal vacuoles accumulate in dystrophic neurites surrounding amyloid β (Aβ) plaques and may be involved in the pathogenesis and progression of Aβ aggregates. Trafficking and thus maturation of these dysfunctional vacuoles is disrupted in the vicinity of Aβ plaques. Transmembrane protein 55B (TMEM55B), also known as phosphatidylinositol-4,5-bisphosphate 4-phosphatase 1 (PIP4P1) is an endo-lysosomal membrane protein that is necessary for appropriate trafficking of endo-lysosomes. The present study tested whether overexpression of TMEM55B in the hippocampus could prevent plaque-associated axonal accumulation of dysfunctional endo-lysosomes, reduce Aβ plaque load, and prevent hippocampal-dependent learning and memory deficits in the 5XFAD mouse models of Aβ plaque pathology. Immunohistochemical analyses revealed a modest but significant reduction in the accumulation of endo-lysosomes in dystrophic neurites surrounding Aβ plaques, but there was no change in hippocampal-dependent memory or plaque load. Overall, these data indicate a potential role for TMEM55B in reducing endo-lysosomal dysfunction during AD-like Aβ pathology.

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.

Nascent Aβ42 Fibrillization in Synaptic Endosomes Precedes Plaque Formation in a Mouse Model of Alzheimer's-like β-Amyloidosis

Accumulation of amyloid-β peptide (Aβ) aggregates in synapses may contribute to the profound synaptic loss characteristic of Alzheimer's disease (AD). The origin of synaptic Aβ aggregates remains elusive, but loss of endosomal proteostasis may trigger their formation. In this study, we identified the synaptic compartments where Aβ accumulates, and performed a longitudinal analysis of synaptosomes isolated from brains of TgCRND8 APP transgenic mice of either sex. To evaluate the specific contribution of Aβ-degrading protease endothelin-converting enzyme (ECE-1) to synaptic/endosomal Aβ homeostasis, we analyzed the effect of partial Ece1 KO in brain and complete ECE1 KO in SH-SY5Y cells. Global inhibition of ECE family members was used to further assess their role in preventing synaptic Aβ accumulation. Results showed that, before extracellular amyloid deposition, synapses were burdened with detergent-soluble Aβ monomers, oligomers, and fibrils. Levels of all soluble Aβ species declined thereafter, as Aβ42 turned progressively insoluble and accumulated in Aβ-producing synaptic endosomal vesicles with characteristics of multivesicular bodies. Accordingly, fibrillar Aβ was detected in brain exosomes. ECE-1-deficient mice had significantly increased endogenous synaptosomal Aβ42 levels, and protease inhibitor experiments showed that, in TgCRND8 mice, synaptic Aβ42 became nearly resistant to degradation by ECE-related proteases. Our study supports that Aβ accumulating in synapses is produced locally, within endosomes, and does not require the presence of amyloid plaques. ECE-1 is a determinant factor controlling the accumulation and fibrillization of nascent Aβ in endosomes and, in TgCRND8 mice, Aβ overproduction causes rapid loss of Aβ42 solubility that curtails ECE-mediated degradation.SIGNIFICANCE STATEMENT Deposition of aggregated Aβ in extracellular plaques is a defining feature of AD. Aβ aggregates also accumulate in synapses and may contribute to the profound synaptic loss and cognitive dysfunction typical of the disease. However, it is not clear whether synaptotoxic Aβ is mainly derived from plaques or if it is produced and aggregated locally, within affected synaptic compartments. Filling this knowledge gap is important for the development of an effective treatment for AD, as extracellular and intrasynaptic pools of Aβ may not be equally modulated by immunotherapies or other therapeutic approaches. In this manuscript, we provide evidence that Aβ aggregates building up in synapses are formed locally, within synaptic endosomes, because of disruptions in nascent Aβ proteostasis.

Protection against Aβ-induced neuronal damage by KU-32: PDHK1 inhibition as important target

A feature of most neurodegenerative diseases is the presence of "mis-folded proteins" that form aggregates, suggesting suboptimal activity of neuronal molecular chaperones. Heat shock protein 90 (Hsp90) is the master regulator of cell responses to "proteotoxic" stresses. Some Hsp90 modulators activate cascades leading to upregulation of additional chaperones. Novobiocin is a modulator at the C-terminal ATP-binding site of Hsp90. Of several novobiocin analogs synthesized and tested for protection against amyloid beta (Aβ)-induced neuronal death, "KU-32" was the most potent in protecting primary neurons, but did not increase expression of other chaperones believed to help clear misfolded proteins. However, KU-32 reversed Aβ-induced superoxide formation, activated Complex I of the electron transfer chain in mitochondria, and blocked the Aβ-induced inhibition of Complex I in neuroblastoma cells. A mechanism for these effects of KU-32 on mitochondrial metabolism appeared to be the inhibition of pyruvate dehydrogenase kinase (PDHK), both in isolated brain mitochondria and in SH-SY5Y cells. PDHK inhibition by the classic enzyme inhibitor, dichloroacetate, led to neuroprotection from Aβ25-35-induced cell injury similarly to KU-32. Inhibition of PDHK in neurons would lead to activation of the PDH complex, increased acetyl-CoA generation, stimulation of the tricarboxylic acid cycle and Complex I in the electron transfer chain, and enhanced oxidative phosphorylation. A focus of future studies may be on the potential value of PDHK as a target in AD therapy.

The Role of Cysteine Protease Cathepsins B, H, C, and X/Z in Neurodegenerative Diseases and Cancer

Papain-like cysteine proteases are composed of 11 human cysteine cathepsins, originally located in the lysosomes. They exhibit broad specificity and act as endopeptidases and/or exopeptidases. Among them, only cathepsins B, H, C, and X/Z exhibit exopeptidase activity. Recently, cysteine cathepsins have been found to be present outside the lysosomes and often participate in various pathological processes. Hence, they have been considered key signalling molecules. Their potentially hazardous proteolytic activities are tightly regulated. This review aims to discuss recent advances in understanding the structural aspects of these four cathepsins, mechanisms of their zymogen activation, regulation of their activities, and functional aspects of these enzymes in neurodegeneration and cancer. Neurodegenerative effects have been evaluated, particularly in Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, multiple sclerosis, and neuropsychiatric disorders. Cysteine cathepsins also participate in tumour progression and metastasis through the overexpression and secretion of proteases, which trigger extracellular matrix degradation. To our knowledge, this is the first review to provide an in-depth analysis regarding the roles of cysteine cathepsins B, H, C, and X in neurodegenerative diseases and cancer. Further advances in understanding the functions of cysteine cathepsins in these conditions will result in the development of novel, targeted therapeutic strategies.

Maternal separation differentially modulates early pathology by sex in 5xFAD Alzheimer's disease-transgenic mice

Alzheimer's disease (AD) is the most common neurodegenerative disease. Most cases of AD are considered idiopathic and likely due to a combination of genetic, environmental, and lifestyle-related risk factors. Despite occurring decades before the typical age of an AD diagnosis, early-life stress (ELS) has been suggested to have long-lasting effects that may contribute to AD risk and pathogenesis. Still, the mechanisms that underlie the role of ELS on AD risk remain largely unknown. Here, we used 5xFAD transgenic mice to study relatively short-term alterations related to ELS in an AD-like susceptible mouse model at 6 weeks of age. To model ELS, we separated pups from their dams for 3 h per day from postnatal day 2-14. Around 6 weeks of age, we found that maternally separated (MS) 5xFAD mice, particularly female mice, displayed increased amyloid-β-immunoreactivity in the anterior cingulate cortex (ACC) and basolateral amygdala (BLA). In anterior cingulate cortex, we also noted significantly increased intraneuronal amyloid-β-immunoreactivity associated with MS but only in female mice. Moreover, IBA1-positive DAPI density was significantly increased in relation to MS in ACC and BLA, and microglia in BLA of MS mice had significantly different morphology compared to microglia in non-MS 5xFAD mice. Cytokine analysis showed that male MS mice, specifically, had increased levels of neuroinflammatory markers CXCL1 and IL-10 in hippocampal extracts compared to non-MS counterparts. Additionally, hippocampal extracts from both male and female MS 5xFAD mice had decreased levels of synapse- and activity-related markers Bdnf, 5htr6, Cox2, and Syp in hippocampus. Lastly, we performed behavioral tests to evaluate anxiety- and depressive-like behavior and working memory but could not detect any significant differences between groups. Overall, we detected several sex-specific molecular and cellular alterations in 6-week-old adolescent 5xFAD mice associated with MS that may help explain the connection between ELS and AD risk.

Cholesterol-dependent amyloid β production: space for multifarious interactions between amyloid precursor protein, secretases, and cholesterol

Amyloid β is considered a key player in the development and progression of Alzheimer's disease (AD). Many studies investigating the effect of statins on lowering cholesterol suggest that there may be a link between cholesterol levels and AD pathology. Since cholesterol is one of the most abundant lipid molecules, especially in brain tissue, it affects most membrane-related processes, including the formation of the most dangerous form of amyloid β, Aβ42. The entire Aβ production system, which includes the amyloid precursor protein (APP), β-secretase, and the complex of γ-secretase, is highly dependent on membrane cholesterol content. Moreover, cholesterol can affect amyloidogenesis in many ways. Cholesterol influences the stability and activity of secretases, but also dictates their partitioning into specific cellular compartments and cholesterol-enriched lipid rafts, where the amyloidogenic machinery is predominantly localized. The most complicated relationships have been found in the interaction between cholesterol and APP, where cholesterol affects not only APP localization but also the precise character of APP dimerization and APP processing by γ-secretase, which is important for the production of Aβ of different lengths. In this review, we describe the intricate web of interdependence between cellular cholesterol levels, cholesterol membrane distribution, and cholesterol-dependent production of Aβ, the major player in AD.

Abnormal accumulation of extracellular vesicles in hippocampal dystrophic axons and regulation by the primary cilia in Alzheimer's disease

Dystrophic neurites (DNs) are abnormal axons and dendrites that are swollen or deformed in various neuropathological conditions. In Alzheimer's disease (AD), DNs play a crucial role in impairing neuronal communication and function, and they may also contribute to the accumulation and spread of amyloid beta (Aβ) in the brain of AD patients. However, it is still a challenge to understand the DNs of specific neurons that are vulnerable to Aβ in the pathogenesis of AD. To shed light on the development of radiating DNs, we examined enriched dystrophic hippocampal axons in a mouse model of AD using a three-dimensional rendering of projecting neurons. We employed the anterograde spread of adeno-associated virus (AAV)1 and conducted proteomic analysis of synaptic compartments obtained from hippocampo-septal regions. Our findings revealed that DNs were formed due to synaptic loss at the axon terminals caused by the accumulation of extracellular vesicle (EV). Abnormal EV-mediated transport and exocytosis were identified in association with primary cilia, indicating their involvement in the accumulation of EVs at presynaptic terminals. To further address the regulation of DNs by primary cilia, we conducted knockdown of the Ift88 gene in hippocampal neurons, which impaired EV-mediated secretion of Aβ and promoted accumulation of axonal spheroids. Using single-cell RNA sequencing, we identified the septal projecting hippocampal somatostatin neurons (SOM) as selectively vulnerable to Aβ with primary cilia dysfunction and vesicle accumulation. Our study suggests that DNs in AD are initiated by the ectopic accumulation of EVs at the neuronal axon terminals, which is affected by neuronal primary cilia.

Antibody-Mediated Clearance of Brain Amyloid-β: Mechanisms of Action, Effects of Natural and Monoclonal Anti-Aβ Antibodies, and Downstream Effects

Immunotherapeutic efforts to slow the clinical progression of Alzheimer's disease (AD) by lowering brain amyloid-β (Aβ) have included Aβ vaccination, intravenous immunoglobulin (IVIG) products, and anti-Aβ monoclonal antibodies. Neither Aβ vaccination nor IVIG slowed disease progression. Despite conflicting phase III results, the monoclonal antibody Aducanumab received Food and Drug Administration (FDA) approval for treatment of AD in June 2021. The only treatments unequivocally demonstrated to slow AD progression to date are the monoclonal antibodies Lecanemab and Donanemab. Lecanemab received FDA approval in January 2023 based on phase II results showing lowering of PET-detectable Aβ; phase III results released at that time indicated slowing of disease progression. Topline results released in May 2023 for Donanemab's phase III trial revealed that primary and secondary end points had been met. Antibody binding to Aβ facilitates its clearance from the brain via multiple mechanisms including promoting its microglial phagocytosis, activating complement, dissolving fibrillar Aβ, and binding of antibody-Aβ complexes to blood-brain barrier receptors. Antibody binding to Aβ in peripheral blood may also promote cerebral efflux of Aβ by a peripheral sink mechanism. According to the amyloid hypothesis, for Aβ targeting to slow AD progression, it must decrease downstream neuropathological processes including tau aggregation and phosphorylation and (possibly) inflammation and oxidative stress. This review discusses antibody-mediated mechanisms of Aβ clearance, findings in AD trials involving Aβ vaccination, IVIG, and anti-Aβ monoclonal antibodies, downstream effects reported in those trials, and approaches which might improve the Aβ-clearing ability of monoclonal antibodies.

Retromer Proteins Reduced in Down Syndrome and the Dp16 Model: Impact of APP Dose and Preclinical Studies of a γ-Secretase Modulator

OBJECTIVE

The retromer complex plays an essential role in intracellular endosomal sorting. Deficits in the retromer complex are linked to enhanced Aβ production. The levels of the components of the retromer complex are reported to be downregulated in Alzheimer disease (AD). Down syndrome (DS) shares neuropathological features with AD. Recent evidence points to dysregulation of the retromer complex in DS. The mechanisms underlying retromer deficits in DS and AD are poorly understood.

METHODS

We measured the levels of retromer components in the frontal cortex of cases of DS-AD (AD in DS) as well as DS; the frontal cortex of a person partially trisomic (PT-DS) for human chromosome 21 (HSA21), whose genome had only the normal 2 copies of the APP gene, was also examined. We also analyzed these proteins in the Dp16 mouse model of DS. To further explore the molecular mechanism for changes in the retromer complex, we treated Dp16 mice with a γ-secretase modulator (GSM; 776890), a treatment that reduces the levels of Aβ42 and Aβ40.

RESULTS

We found VPS26A, VPS26B, and VPS29, but not VPS35, were significantly reduced in both DS and DS-AD, but not in PT-DS. Downregulation of VPS26A, VPS26B, and VPS29 was recapitulated in the brains of old Dp16 mice (at 16 months of age) and required increased App gene dose. Significantly, GSM treatment completely prevented reductions of the retromer complex.

INTERPRETATION

Our studies point to increased APP gene dose as a compromising retromer function in DS and suggest a causal role for Aβ42 and Aβ40. ANN NEUROL 2023;94:245-258.

Lysosomal dysfunction in Down syndrome and Alzheimer mouse models is caused by v-ATPase inhibition by Tyr682-phosphorylated APP βCTF

Lysosome dysfunction arises early and propels Alzheimer's disease (AD). Herein, we show that amyloid precursor protein (APP), linked to early-onset AD in Down syndrome (DS), acts directly via its β-C-terminal fragment (βCTF) to disrupt lysosomal vacuolar (H+)-adenosine triphosphatase (v-ATPase) and acidification. In human DS fibroblasts, the phosphorylated 682YENPTY internalization motif of APP-βCTF binds selectively within a pocket of the v-ATPase V0a1 subunit cytoplasmic domain and competitively inhibits association of the V1 subcomplex of v-ATPase, thereby reducing its activity. Lowering APP-βCTF Tyr682 phosphorylation restores v-ATPase and lysosome function in DS fibroblasts and in vivo in brains of DS model mice. Notably, lowering APP-βCTF Tyr682 phosphorylation below normal constitutive levels boosts v-ATPase assembly and activity, suggesting that v-ATPase may also be modulated tonically by phospho-APP-βCTF. Elevated APP-βCTF Tyr682 phosphorylation in two mouse AD models similarly disrupts v-ATPase function. These findings offer previously unknown insight into the pathogenic mechanism underlying faulty lysosomes in all forms of AD.

A bioinformatic investigation of proteasome and autophagy expression in the central nervous system

The ubiquitin proteasome system (UPS) and autophagy lysosome pathway (ALP) are crucial in the control of protein quality. However, data regarding the relative significance of UPS and ALP in the central nervous system (CNS) are limited. In the present study, using publicly available data, we computed the quantitative expression status of UPS- and ALP-related genes and their products in the CNS as compared with that in other tissues and cells. We obtained human and mouse gene expression datasets from the reference expression dataset (RefEx) and Genevestigator (a tool for handling curated transcriptomic data from public repositories) as well as human proteomics data from the proteomics database (ProteomicsDB). The expression levels of genes and proteins in four categories-ubiquitin, proteasome, autophagy, and lysosome--in the cells and tissues were assessed. Perturbation of the gene expression by drugs was also analyzed for the four categories. Compared with that for ubiquitin, autophagy, and lysosome, gene expression for proteasome was consistently lower in the CNS of mice but was more pronounced in humans. Neural stem cells and neurons showed low proteasome gene expression as compared with embryonic stem cells. Proteomic analyses, however, did not show trends similar to those observed in the gene expression analyses. Perturbation analyses revealed that azithromycin and vitamin D3 upregulated the expression of both UPS and ALP. Gene and proteomic expression data could offer a fresh perspective on CNS pathophysiology. Our results indicate that disproportional expression of UPS and ALP might affect CNS disorders and that this imbalance might be redressed by several therapeutic candidates.

Human microglia show unique transcriptional changes in Alzheimer's disease

Microglia, the innate immune cells of the brain, influence Alzheimer's disease (AD) progression and are potential therapeutic targets. However, microglia exhibit diverse functions, the regulation of which is not fully understood, complicating therapeutics development. To better define the transcriptomic phenotypes and gene regulatory networks associated with AD, we enriched for microglia nuclei from 12 AD and 10 control human dorsolateral prefrontal cortices (7 males and 15 females, all aged >60 years) before single-nucleus RNA sequencing. Here we describe both established and previously unrecognized microglial molecular phenotypes, the inferred gene networks driving observed transcriptomic change, and apply trajectory analysis to reveal the putative relationships between microglial phenotypes. We identify microglial phenotypes more prevalent in AD cases compared with controls. Further, we describe the heterogeneity in microglia subclusters expressing homeostatic markers. Our study demonstrates that deep profiling of microglia in human AD brain can provide insight into microglial transcriptional changes associated with AD.

Converging links between adult-onset neurodegenerative Alzheimer's disease and early life neurodegenerative neuronal ceroid lipofuscinosis?

Evidence from genetics and from analyzing cellular and animal models have converged to suggest links between neurodegenerative disorders of early and late life. Here, we summarize emerging links between the most common late life neurodegenerative disease, Alzheimer's disease, and the most common early life neurodegenerative diseases, neuronal ceroid lipofuscinoses. Genetic studies reported an overlap of clinically diagnosed Alzheimer's disease and mutations in genes known to cause neuronal ceroid lipofuscinoses. Accumulating data strongly suggest dysfunction of intracellular trafficking mechanisms and the autophagy-endolysosome system in both types of neurodegenerative disorders. This suggests shared cytopathological processes underlying these different types of neurodegenerative diseases. A better understanding of the common mechanisms underlying the different diseases is important as this might lead to the identification of novel targets for therapeutic concepts, the transfer of therapeutic strategies from one disease to the other and therapeutic approaches tailored to patients with specific mutations. Here, we review dysfunctions of the endolysosomal autophagy pathway in Alzheimer's disease and neuronal ceroid lipofuscinoses and summarize emerging etiologic and genetic overlaps.

Defective lysosomal acidification: a new prognostic marker and therapeutic target for neurodegenerative diseases

Lysosomal acidification dysfunction has been implicated as a key driving factor in the pathogenesis of neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. Multiple genetic factors have been linked to lysosomal de-acidification through impairing the vacuolar-type ATPase and ion channels on the organelle membrane. Similar lysosomal abnormalities are also present in sporadic forms of neurodegeneration, although the underlying pathogenic mechanisms are unclear and remain to be investigated. Importantly, recent studies have revealed early occurrence of lysosomal acidification impairment before the onset of neurodegeneration and late-stage pathology. However, there is a lack of methods for organelle pH monitoring in vivo and a dearth of lysosome-acidifying therapeutic agents. Here, we summarize and present evidence for the notion of defective lysosomal acidification as an early indicator of neurodegeneration and urge the critical need for technological advancement in developing tools for lysosomal pH monitoring and detection both in vivo and for clinical applications. We further discuss current preclinical pharmacological agents that modulate lysosomal acidification, including small molecules and nanomedicine, and their potential clinical translation into lysosome-targeting therapies. Both timely detection of lysosomal dysfunction and development of therapeutics that restore lysosomal function represent paradigm shifts in targeting neurodegenerative diseases.

The triggering receptor expressed on myeloid cells 2-apolipoprotein E signaling pathway in diseases

ABSTRACT

Triggering receptor expressed on myeloid cells 2 (TREM2) is a membrane receptor on myeloid cells and plays an important role in the body's immune defense. Recently, TREM2 has received extensive attention from researchers, and its activity has been found in Alzheimer's disease, neuroinflammation, and traumatic brain injury. The appearance of TREM2 is usually accompanied by changes in apolipoprotein E (ApoE), and there has been a lot of research into their structure, as well as the interaction mode and signal pathways involved in them. As two molecules with broad and important roles in the human body, understanding their correlation may provide therapeutic targets for certain diseases. In this article, we reviewed several diseases in which TREM2 and ApoE are synergistically involved in the development. We further discussed the positive or negative effects of the TREM2-ApoE pathway on nervous system immunity and inflammation.

The Cleavage-Specific Tau 12A12mAb Exerts an Anti-Amyloidogenic Action by Modulating the Endocytic and Bioenergetic Pathways in Alzheimer's Disease Mouse Model

Beyond deficits in hippocampal-dependent episodic memory, Alzheimer's Disease (AD) features sensory impairment in visual cognition consistent with extensive neuropathology in the retina. 12A12 is a monoclonal cleavage specific antibody (mAb) that in vivo selectively neutralizes the AD-relevant, harmful N-terminal 20-22 kDa tau fragment(s) (i.e., NH₂htau) without affecting the full-length normal protein. When systemically injected into the Tg2576 mouse model overexpressing a mutant form of Amyloid Precursor Protein (APP), APPK670/671L linked to early onset familial AD, this conformation-specific tau mAb successfully reduces the NH₂htau accumulating both in their brain and retina and, thus, markedly alleviates the phenotype-associated signs. By means of a combined biochemical and metabolic experimental approach, we report that 12A12mAb downregulates the steady state expression levels of APP and Beta-Secretase 1 (BACE-1) and, thus, limits the Amyloid beta (Aβ) production both in the hippocampus and retina from this AD animal model. The local, antibody-mediated anti-amyloidogenic action is paralleled in vivo by coordinated modulation of the endocytic (BIN1, RIN3) and bioenergetic (glycolysis and L-Lactate) pathways. These findings indicate for the first time that similar molecular and metabolic retino-cerebral pathways are modulated in a coordinated fashion in response to 12A12mAb treatment to tackle the neurosensorial Aβ accumulation in AD neurodegeneration.

IGF2 in memory, neurodevelopmental disorders, and neurodegenerative diseases

Insulin-like growth factor 2 (IGF2) emerged as a critical mechanism of synaptic plasticity and learning and memory. Deficits in IGF2 in the brain, serum, or cerebrospinal fluid (CSF) are associated with brain diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS). Increasing IGF2 levels enhances memory in healthy animals and reverses numerous symptoms in laboratory models of aging, neurodevelopmental disorders, and neurodegenerative diseases. These effects occur via the IGF2 receptor (IGF2R) - a receptor that is highly expressed in neurons and regulates protein trafficking, synthesis, and degradation. Here, I summarize the current knowledge regarding IGF2 expression and functions in the brain, particularly in memory, and propose a novel conceptual model for IGF2/IGF2R mechanisms of action in brain health and diseases.

Multi-omic approach characterises the neuroprotective role of retromer in regulating lysosomal health

Retromer controls cellular homeostasis through regulating integral membrane protein sorting and transport and by controlling maturation of the endo-lysosomal network. Retromer dysfunction, which is linked to neurodegenerative disorders including Parkinson's and Alzheimer's diseases, manifests in complex cellular phenotypes, though the precise nature of this dysfunction, and its relation to neurodegeneration, remain unclear. Here, we perform an integrated multi-omics approach to provide precise insight into the impact of Retromer dysfunction on endo-lysosomal health and homeostasis within a human neuroglioma cell model. We quantify widespread changes to the lysosomal proteome, indicative of broad lysosomal dysfunction and inefficient autophagic lysosome reformation, coupled with a reconfigured cell surface proteome and secretome reflective of increased lysosomal exocytosis. Through this global proteomic approach and parallel transcriptomic analysis, we provide a holistic view of Retromer function in regulating lysosomal homeostasis and emphasise its role in neuroprotection.

Reverse electron transfer is activated during aging and contributes to aging and age-related disease

Mechanisms underlying the depletion of NAD+ and accumulation of reactive oxygen species (ROS) in aging and age-related disorders remain poorly defined. We show that reverse electron transfer (RET) at mitochondrial complex I, which causes increased ROS production and NAD+ to NADH conversion and thus lowered NAD+ /NADH ratio, is active during aging. Genetic or pharmacological inhibition of RET decreases ROS production and increases NAD+ /NADH ratio, extending the lifespan of normal flies. The lifespan-extending effect of RET inhibition is dependent on NAD+ -dependent Sirtuin, highlighting the importance of NAD+ /NADH rebalance, and on longevity-associated Foxo and autophagy pathways. RET and RET-induced ROS and NAD+ /NADH ratio changes are prominent in human induced pluripotent stem cell (iPSC) model and fly models of Alzheimer's disease (AD). Genetic or pharmacological inhibition of RET prevents the accumulation of faulty translation products resulting from inadequate ribosome-mediated quality control, rescues relevant disease phenotypes, and extends the lifespan of Drosophila and mouse AD models. Deregulated RET is therefore a conserved feature of aging, and inhibition of RET may open new therapeutic opportunities in the context of aging and age-related diseases including AD.

The synthetic TRPML1 agonist ML-SA1 rescues Alzheimer-related alterations of the endosomal-autophagic-lysosomal system

Abnormalities in the endosomal-autophagic-lysosomal (EAL) system are an early event in Alzheimer's disease (AD) pathogenesis. However, the mechanisms underlying these abnormalities are unclear. The transient receptor potential channel mucolipin 1(TRPML1, also known as MCOLN1), a vital endosomal-lysosomal Ca2+ channel whose loss of function leads to neurodegeneration, has not been investigated with respect to EAL pathogenesis in late-onset AD (LOAD). Here, we identify pathological hallmarks of TRPML1 dysregulation in LOAD neurons, including increased perinuclear clustering and vacuolation of endolysosomes. We reveal that induced pluripotent stem cell (iPSC)-derived human cortical neurons expressing APOE ε4, the strongest genetic risk factor for LOAD, have significantly diminished TRPML1-induced endolysosomal Ca2+ release. Furthermore, we found that blocking TRPML1 function in primary neurons by depleting the TRPML1 agonist PI(3,5)P2 via PIKfyve inhibition, recreated multiple features of EAL neuropathology evident in LOAD. This included increased endolysosomal Ca2+ content, enlargement and perinuclear clustering of endolysosomes, autophagic vesicle accumulation and early endosomal enlargement. Strikingly, these AD-like neuronal EAL defects were rescued by TRPML1 reactivation using its synthetic agonist ML-SA1. These findings implicate defects in TRPML1 in LOAD EAL pathogenesis and present TRPML1 as a potential therapeutic target.

Lysosomal dysfunction induced cytosolic vacuolation and increased intracellular amyloid-beta 42 (Aβ42) in human brain endothelial cells (HBEC-5i)

Lysosome is a primary degradative organelle and is crucial in cellular homeostasis. A reduction in its function due to ageing has been associated with the development of Alzheimer's disease (AD), a common neurodegenerative disorder characterised by the deposition of neurotoxic amyloid plaque in the brain and cerebral vessel walls. The breakdown of the blood-brain barrier (BBB) plays a vital role in the pathogenesis of AD. However, the impact of lysosomal dysfunction on brain endothelial cells, the key component of the BBB, in the disease progression is yet to be fully understood. In this study, human brain endothelial cells (HBEC-5i) were exposed to a lysosomotropic compound, chloroquine (CQ) for 24 h. Cell viability was assessed with the 3-(4,5-dimethylthiazol-2-yl)- 2,5-diphenyltetrazolium bromide (MTT) assay to determine the inhibitory concentration (IC) at IC₁₀ (17.5 µM), IC₂₅ (70.5 µM), and IC₅₀ (125 µM). The morphological changes observed include vacuoles arrested in the cytosols and cell shrinkage that were more prominent at IC₂₅ and IC₅₀. Lysosomal dysfunction was evaluated by measuring the lysosomal-associated membrane protein-1 (LAMP-1) and microtubule-associated protein light chain 3-II (LC3-II) using the capillary-based immunoassay. LC3-II was significantly increased at IC₂₅ and IC₅₀ (p < 0.05 and p < 0.001_, respectively). The concentration of intracellular and extracellular Aβ42 was quantitated using the enzyme-linked immunosorbent assay, which demonstrated a significant increase (p < 0.05) in intracellular Aβ42 at IC₂₅. This study showed that perturbation of lysosomal function impairs autophagy that leads to intracellular increment of Aβ, indicating the important roles of lysosomes in endothelial cells homeostasis and disease progression.

CREB3L2-ATF4 heterodimerization defines a transcriptional hub of Alzheimer's disease gene expression linked to neuropathology

Gene expression is changed by disease, but how these molecular responses arise and contribute to pathophysiology remains less understood. We discover that β-amyloid, a trigger of Alzheimer's disease (AD), promotes the formation of pathological CREB3L2-ATF4 transcription factor heterodimers in neurons. Through a multilevel approach based on AD datasets and a novel chemogenetic method that resolves the genomic binding profile of dimeric transcription factors (ChIPmera), we find that CREB3L2-ATF4 activates a transcription network that interacts with roughly half of the genes differentially expressed in AD, including subsets associated with β-amyloid and tau neuropathologies. CREB3L2-ATF4 activation drives tau hyperphosphorylation and secretion in neurons, in addition to misregulating the retromer, an endosomal complex linked to AD pathogenesis. We further provide evidence for increased heterodimer signaling in AD brain and identify dovitinib as a candidate molecule for normalizing β-amyloid-mediated transcriptional responses. The findings overall reveal differential transcription factor dimerization as a mechanism linking disease stimuli to the development of pathogenic cellular states.

Rab3A interacts with spastin to regulate neurite outgrowth in hippocampal neurons

Investigating novel mechanisms of neurite outgrowth via cytoskeleton is critical for developing therapeutic strategies against neural disorders. Rab3A is a vesicle-related protein distributed throughout the nervous system, but the detailed mechanism related to cytoskeleton remains largely unknown. Our previous reports show that spastin serves microtubule to regulate neurite outgrowth. Here, we asked whether Rab3A could function via modulating spastin during neuronal development. The results revealed that Rab3A colocalized with spastin in cultured hippocampal neurons. Immunoprecipitation assays showed that Rab3A physically interacted with spastin in rat brain lysates. Rab3A overexpression significantly induced spastin degradation; this effect was reversed by leupeptin- or MG-132- administration, suggesting the lysosomal and ubiquitin-mediated degradation system. Immunofluorescence staining further confirmed that Rab3A and spastin immune-colocalized with the lysosome marker lysotracker. In COS7 cells, Rab3A overexpression significantly downregulated spastin expression and abolished the spastin-mediated microtubule severing. Furthermore, overexpression inhibited while genetic knockdown of Rab3A promoted neurite outgrowth. However, this inhibitory effect on neurite outgrowth in hippocampal neurons could be reversed via co-transfection of spastin, indicating that Rab3A functions via its interaction protein spastin. In general, our data identify an interaction between Rab3A and spastin, and this interaction affects the protein stability of spastin and eliminates its microtubule severing function, thereby modulating neurite outgrowth.

Genome-wide association study of brain biochemical phenotypes reveals distinct genetic architecture of Alzheimer's disease related proteins

BACKGROUND

Alzheimer's disease (AD) is neuropathologically characterized by amyloid-beta (Aβ) plaques and neurofibrillary tangles. The main protein components of these hallmarks include Aβ40, Aβ42, tau, phosphor-tau, and APOE. We hypothesize that genetic variants influence the levels and solubility of these AD-related proteins in the brain; identifying these may provide key insights into disease pathogenesis.

METHODS

Genome-wide genotypes were collected from 441 AD cases, imputed to the haplotype reference consortium (HRC) panel, and filtered for quality and frequency. Temporal cortex levels of five AD-related proteins from three fractions, buffer-soluble (TBS), detergent-soluble (Triton-X = TX), and insoluble (Formic acid = FA), were available for these same individuals. Variants were tested for association with each quantitative biochemical measure using linear regression, and GSA-SNP2 was used to identify enriched Gene Ontology (GO) terms. Implicated variants and genes were further assessed for association with other relevant variables.

RESULTS

We identified genome-wide significant associations at seven novel loci and the APOE locus. Genes and variants at these loci also associate with multiple AD-related measures, regulate gene expression, have cell-type specific enrichment, and roles in brain health and other neuropsychiatric diseases. Pathway analysis identified significant enrichment of shared and distinct biological pathways.

CONCLUSIONS

Although all biochemical measures tested reflect proteins core to AD pathology, our results strongly suggest that each have unique genetic architecture and biological pathways that influence their specific biochemical states in the brain. Our novel approach of deep brain biochemical endophenotype GWAS has implications for pathophysiology of proteostasis in AD that can guide therapeutic discovery efforts focused on these proteins.

Impaired autophagy in amyloid-beta pathology: A traditional review of recent Alzheimer's research

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder. The major pathological changes in AD progression are the generation and accumulation of amyloid-beta (Aβ) peptides as well as the presence of abnormally hyperphosphorylated tau proteins in the brain. Autophagy is a conserved degradation pathway that eliminates abnormal protein aggregates and damaged organelles. Previous studies have suggested that autophagy plays a key role in the production and clearance of Aβ peptides to maintain a steady-state of Aβ peptides levels. However, a growing body of evidence suggests that autophagy is significantly impaired in the pathogenesis of AD, especially in Aβ metabolism. Therefore, this article reviews the latest studies concerning the mechanisms of autophagy, the metabolism of Aβ peptides, and the defective autophagy in the production and clearance of Aβ peptides. Here, we also summarize the established and new strategies for targeting autophagy in vivo and through clinical AD trials to identify gaps in our knowledge and to generate further questions.

Biomolecular Markers of Brain Aging

Characterized by the gradual loss of physiological integrity, impaired function, and increased susceptibility to death, aging is considered the primary risk factor for major human diseases, such as cancer, diabetes, cardiovascular disorders, and neurodegenerative diseases. The time-dependent accumulation of cellular damage is widely considered the general cause of aging. While the mechanism of normal aging is still unresolved, researchers have identified different markers of aging, including genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient-sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. Theories of aging can be divided into two categories: (1) aging is a genetically programmed process, and (2) aging is a random process caused by gradual damage to the organism over time as a result of its vital activities. Aging affects the entire human body, and aging of the brain is undoubtedly different from all other organs, as neurons are highly differentiated postmitotic cells, and the lifespan of most neurons in the postnatal period is equal to the lifespan of the brain. In this chapter, we discuss the conserved mechanisms of aging that may underlie the changes observed in the aging brain, with a focus on mitochondrial function and oxidative stress, autophagy and protein turnover, insulin/IGF signaling, target of rapamycin (TOR) signaling, and sirtuin function.

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.

Modelling Alzheimer's disease using human brain organoids: current progress and challenges

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterised by gradual memory loss and declining cognitive and executive functions. AD is the most common cause of dementia, affecting more than 50 million people worldwide, and is a major health concern in society. Despite decades of research, the cause of AD is not well understood and there is no effective curative treatment so far. Therefore, there is an urgent need to increase understanding of AD pathophysiology in the hope of developing a much-needed cure. Dissecting the cellular and molecular mechanisms of AD pathogenesis has been challenging as the most commonly used model systems such as transgenic animals and two-dimensional neuronal culture do not fully recapitulate the pathological hallmarks of AD. The recent advent of three-dimensional human brain organoids confers unique opportunities to study AD in a humanised model system by encapsulating many aspects of AD pathology. In the present review, we summarise the studies of AD using human brain organoids that recapitulate the major pathological components of AD including amyloid-β and tau aggregation, neuroinflammation, mitochondrial dysfunction, oxidative stress and synaptic and circuitry dysregulation. Additionally, the current challenges and future directions of the brain organoids modelling system are discussed.

Altered glucose metabolism in Alzheimer's disease: Role of mitochondrial dysfunction and oxidative stress

Increasing evidence suggests that abnormal cerebral glucose metabolism is largely present in Alzheimer's disease (AD). The brain utilizes glucose as its main energy source and a decline in its metabolism directly reflects on brain function. Weighing on recent evidence, here we systematically assessed the aberrant glucose metabolism associated with amyloid beta and phosphorylated tau accumulation in AD brain. Interlink between insulin signaling and AD highlighted the involvement of the IRS/PI3K/Akt/AMPK signaling, and GLUTs in the disease progression. While shedding light on the mitochondrial dysfunction in the defective glucose metabolism, we further assessed functional consequences of AGEs (advanced glycation end products) accumulation, polyol activation, and other contributing factors including terminal respiration, ROS (reactive oxygen species), mitochondrial permeability, PINK1/parkin defects, lysosome-mitochondrial crosstalk, and autophagy/mitophagy. Combined with the classic plaque and tangle pathologies, glucose hypometabolism with acquired insulin resistance and mitochondrial dysfunction potentiate these factors to exacerbate AD pathology. To this end, we further reviewed AD and DM (diabetes mellitus) crosstalk in disease progression. Taken together, the present work discusses the emerging role of altered glucose metabolism, contributing impact of insulin signaling, and mitochondrial dysfunction in the defective cerebral glucose utilization in AD.