Differential regulation of amyloid-β endocytic trafficking and lysosomal degradation by apolipoprotein E isoforms

sAPPα Peptide Promotes Damaged Microglia to Clear Alzheimer's Amyloid-β via Restoring Mitochondrial Function

Alzheimer's disease (AD) is an age-related neurodegenerative disease with amyloid-β (Aβ) deposition as the main pathological feature. It's an important challenge to find new ways to clear Aβ from the brain. The soluble amyloid precursor protein α (sAPPα) is a neuroprotective protein and can attenuate neuronal damage, including toxic Aβ. However, the regulatory role of sAPPα in non-neuronal cells, such as microglia, is less reported and controversial. Here, we showed that sAPPα promoted the phagocytosis and degradation of Aβ in both normal and damaged microglia. Moreover, the function of damaged microglia was improved by the sAPPα through normalizing mitochondrial function. Furthermore, the results of molecular docking simulation showed that sAPPα had a good affinity with Aβ. We preliminarily reveal that sAPPα is similar to antibodies and can participate in the regulation of microglia phagocytosis and degradation of Aβ after binding to Aβ. sAPPα is expected to be a mild and safe peptide drug or drug carrier for AD.

Cell type-specific functions of Alzheimer's disease endocytic risk genes

Endocytosis is a key cellular pathway required for the internalization of cellular nutrients, lipids and receptor-bound cargoes. It is also critical for the recycling of cellular components, cellular trafficking and membrane dynamics. The endocytic pathway has been consistently implicated in Alzheimer's disease (AD) through repeated genome-wide association studies and the existence of rare coding mutations in endocytic genes. BIN1 and PICALM are two of the most significant late-onset AD risk genes after APOE and are both key to clathrin-mediated endocytic biology. Pathological studies also demonstrate that endocytic dysfunction is an early characteristic of late-onset AD, being seen in the prodromal phase of the disease. Different cell types of the brain have specific requirements of the endocytic pathway. Neurons require efficient recycling of synaptic vesicles and microglia use the specialized form of endocytosis-phagocytosis-for their normal function. Therefore, disease-associated changes in endocytic genes will have varied impacts across different cell types, which remains to be fully explored. Given the genetic and pathological evidence for endocytic dysfunction in AD, understanding how such changes and the related cell type-specific vulnerabilities impact normal cellular function and contribute to disease is vital and could present novel therapeutic opportunities. This article is part of a discussion meeting issue 'Understanding the endo-lysosomal network in neurodegeneration'.

Rab11 promotes single Mauthner cell axon regeneration in vivo through axon guidance molecule Ntng2b

Effective axon regeneration within the central nervous system (CNS) is pivotal for achieving functional recovery following spinal cord injury (SCI). Numerous extrinsic and intrinsic factors exert influences on the axon regeneration. While prior studies have demonstrated crucial involvement of specific members the Rab protein family in axon regeneration in the peripheral nervous system (PNS), the precise function of Rab11 in CNS axon regeneration in vivo remains elusive. Thus, our study aimed to elucidate the impact of Rab11 on the axon regeneration of Mauthner cells (M-cells) in zebrafish larvae. Our findings demonstrated that overexpression of Rab11bb via single-cell electroporation significantly promoted axon regeneration in individual M-cells. Conversely, knockdown of Rab11bb inhibited the axon regeneration of M-cells. RNA-seq analysis revealed an upregulation of ntng2b following Rab11bb overexpression. As we hypothesized, overexpression of Ntng2b markedly enhanced axon regeneration, while Ntng2b knockdown in the context of Rab11bb pro-regeneration substantially hindered axon regrowth. In conclusion, our study demonstrated that Rab11 promotes axon regeneration of single M-cell in the CNS through the Rab11/axon guidance/Ntng2b pathway.

The exocyst subunit Sec15 is critical for proper synaptic development and function at the Drosophila NMJ

The exocyst protein complex is important for targeted vesicle fusion in a variety of cell types, however, its function in neurons is still not entirely known. We found that presynaptic knockdown (KD) of the exocyst component sec15 by transgenic RNAi expression caused a number of unexpected morphological and physiological defects in the synapse. These include the development of active zones (AZ) devoid of essential presynaptic proteins, an increase in the branching of the presynaptic arbor, the appearance of satellite boutons, and a decrease in the amplitude of stimulated postsynaptic currents as well as a decrease in the frequency of spontaneous synaptic vesicle release. We also found the release of extracellular vesicles from the presynaptic neuron was greatly diminished in the Sec15 KDs. These effects were mimicked by presynaptic knockdown of Rab11, a protein known to interact with the exocyst. sec15 RNAi expression caused an increase in phosphorylated Mothers against decapentaplegic (pMad) in the presynaptic terminal, an indication of enhanced bone morphogenic protein (BMP) signaling. Some morphological phenotypes caused by Sec15 knockdown were reduced by attenuation of BMP signaling through knockdown of wishful thinking (Wit), while other phenotypes were unaffected. Individual knockdown of multiple proteins of the exocyst complex also displayed a morphological phenotype similar to Sec15 KD. We conclude that Sec15, functioning as part of the exocyst complex, is critically important for proper formation and function of neuronal synapses. We propose a model in which Sec15 is involved in the trafficking of vesicles from the recycling endosome to the cell membrane as well as possibly trafficking extracellular vesicles for presynaptic release and these processes are necessary for the correct structure and function of the synapse.

Rescue of ApoE4-related lysosomal autophagic failure in Alzheimer's disease by targeted small molecules

Homozygosity for the ε4 allele of APOE increases the odds of developing Alzheimer's by 12 to 15 times relative to the most common ε3;ε3 genotype, and its association with higher plaque loads comports with evidence that APOEε4 compromises autophagy. The ApoE4 protein specifically binds a cis element ("CLEAR") in the promoters of several autophagy genes to block their transcription. We used a multifaceted approach to identify a druggable site in ApoE4, and virtual screening of lead-like compounds identified small molecules that specifically bind to this site to impede ApoE4::DNA binding. We validated these molecules both in vitro and in vivo with models expressing ApoE4, including ApoE4 targeted-replacement mice. One compound was able to significantly restore transcription of several autophagy genes and protected against amyloid-like aggregation in a C. elegans AD model. Together, these findings provide proof-of-principle evidence for pharmacological remediation of lysosomal autophagy by ApoE4 via ApoE4-targeted lead molecules that represent a novel tack on neurodegenerative disorders.

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.

Microtubule acetylation induced by oxidative stress regulates subcellular distribution of lysosomal vesicles for amyloid-beta secretion

Excessive production and accumulation of amyloid-beta (Aβ) in the brain are one of the hallmarks of Alzheimer's disease (AD). Although oxidative stress is known to trigger and promote the progression of AD, the molecular relationship between oxidative stress and Aβ production is not yet fully understood. In this study, we demonstrate that microtubule acetylation induced by oxidative stress plays a critical role in Aβ production and secretion by altering the subcellular distribution of Aβ precursor protein (APP)-containing lysosomal vesicles. Under oxidative stress, both H4-APPSwe/Ind and HEK293T-APPSwe/Ind cell lines showed increased microtubule acetylation and Aβ secretion. Knockdown (KD) of alpha-tubulin N-acetyltransferase 1 (ATAT1) by using a lentiviral shRNA not only inhibited the generation of intermediate APP fragments, such as β-CTF and AICD, but also suppressed Aβ secretion. Oxidative stress promoted the dispersion of LAMP1-positive vesicles to the periphery of the cell through microtubule acetylation, leading to the formation of neutralized lysosomal vesicles (NLVs), which was inhibited by ATAT1 KD. Treatment of the cells with the dynein ATPase inhibitor EHNA or downregulation of LIS1, a regulator of dynein-mediated intracellular transport, increased the peripheral localization of NLVs and promoted Aβ secretion, whereas KD of ADP ribosylation factor like GTPase 8B showed the opposite result. ATAT1 KD in the hippocampal region of the 5×FAD AD mouse model also showed significant reductions in Aβ plaque accumulation and memory loss. Taken together, these findings suggest that oxidative stress-induced microtubule acetylation promotes the peripheral localization of lysosomal vesicles to form NLVs, thereby enhancing Aβ secretion.

Amyloid-Beta Peptides 40 and 42 Employ Distinct Molecular Pathways for Cell Entry and Intracellular Transit at the Blood-Brain Barrier Endothelium

The blood-brain barrier (BBB) plays a critical role in maintaining the equilibrium between amyloid beta (Aβ) levels in blood and the brain by regulating Aβ transport. Our previous publications demonstrated that BBB trafficking of Aβ42 and Aβ40 is distinct and is disrupted under various pathophysiological conditions. However, the intracellular mechanisms that allow BBB endothelium to differentially handle Aβ40 and Aβ42 have not been clearly elucidated. In this study, we identified mechanisms of Aβ endocytosis in polarized human cerebral microvascular endothelial cell monolayers. Our studies demonstrated that Aβ peptides with fluorescent label (F-Aβ) were internalized by BBB endothelial cells via energy, dynamin, and actin-dependent endocytosis. Interestingly, endocytosis of F-Aβ40 but not F-Aβ42 was substantially reduced by clathrin inhibition, whereas F-Aβ42 but not F-Aβ40 endocytosis was reduced by half after inhibiting the caveolae-mediated pathway. Following endocytosis, both isoforms were sorted by the endo-lysosomal system. Although Aβ42 was shown to accumulate more in the lysosomes, which could lead to its higher degradation and/or aggregation at lower lysosomal pH, Aβ40 demonstrated robust accumulation in recycling endosomes, which may facilitate its exocytosis by the endothelial cells. These results provide a mechanistic insight into the selective ability of BBB endothelium to transport Aβ40 versus Aβ42. This knowledge contributes to the understanding of molecular pathways underlying Aβ accumulation in the BBB endothelium and associated BBB dysfunction. Moreover, it allows us to establish mechanistic rationale for altered Aβ40:Aβ42 ratios and anomalous amyloid deposition in the cerebral vasculature as well as brain parenchyma during Alzheimer's disease progression. SIGNIFICANCE STATEMENT: Differential interaction of Aβ40 and Aβ42 isoforms with the blood-brain barrier (BBB) endothelium may contribute to perturbation in Aβ42:Aβ40 ratio, which is associated with Alzheimer's disease (AD) progression and severity. The current study identified distinct molecular pathways by which Aβ40 and Aβ42 are trafficked at the BBB, which regulates equilibrium between blood and brain Aβ levels. These findings provide molecular insights into mechanisms that engender BBB dysfunction and promote Aβ accumulation in AD brain.

Lysosomal proteomics reveals mechanisms of neuronal apoE4associated lysosomal dysfunction

ApoE4 is the primary risk factor for Alzheimer's Disease. While apoE is primarily expressed by astrocytes, AD pathology including endosomal abnormalities and mitochondrial dysfunction first occurs in neurons. Lysosomes are poised at the convergence point between these features. We find that apoE4-expressing cells exhibit lysosomal alkalinization, reduced lysosomal proteolysis, and impaired mitophagy. To identify driving factors for this lysosomal dysfunction, we performed quantitative lysosomal proteome profiling. This revealed that apoE4 expression results in lysosomal depletion of Lgals3bp and accumulation of Tmed5 in both Neuro-2a cells and postmitotic human neurons. Modulating the expression of both proteins affected lysosomal function, with Tmed5 knockdown rescuing lysosomal alkalinization in apoE4 cells, and Lgals3bp knockdown causing lysosomal alkalinization and reduced lysosomal density in apoE3 cells. Taken together, our work reveals that apoE4 exerts gain-of-toxicity by alkalinizing the lysosomal lumen, pinpointing lysosomal Tmed5 accumulation and Lgals3bp depletion as apoE4-associated drivers for this phenotype.

Leveraging Plasma Membrane Repair Therapeutics for Treating Neurodegenerative Diseases

Plasma membrane repair is an essential cellular mechanism that reseals membrane disruptions after a variety of insults, and compromised repair capacity can contribute to the progression of many diseases. Neurodegenerative diseases are marked by membrane damage from many sources, reduced membrane integrity, elevated intracellular calcium concentrations, enhanced reactive oxygen species production, mitochondrial dysfunction, and widespread neuronal death. While the toxic intracellular effects of these changes in cellular physiology have been defined, the specific mechanism of neuronal death in certain neurodegenerative diseases remains unclear. An abundance of recent evidence indicates that neuronal membrane damage and pore formation in the membrane are key contributors to neurodegenerative disease pathogenesis. In this review, we have outlined evidence supporting the hypothesis that membrane damage is a contributor to neurodegenerative diseases and that therapeutically enhancing membrane repair can potentially combat neuronal death.

Amyloid futures in the expanding pathology of brain aging and dementia

Positron emission tomography (PET) imaging studies of Alzheimer's disease (AD) patients show progressive increases of fibrillar Aβ-amyloid. Because current PET ligands underestimate nonfibrillar forms, we assayed soluble Aβ in AD and controls. To identify the mechanisms responsible for soluble Aβ in AD brains, we examined lipid rafts (LRs), where amyloid precursor protein (APP) is enzymatically processed. Frontal cortex was compared with cerebellum, which has minimal AD pathology. Compared with cognitively normal controls (CTL; Braak 0-1), elevations of soluble Aβ40 and Aβ42 were similar for intermediate- and later-stage AD (Braak 2-3 and 4-6). Clinical-grade AD showed a greater increase in soluble Aβ40 than Aβ42 relative to CTL. LR raft yield per gram AD frontal cortex was 20% below that of controls, whereas cerebellar LR did not differ by Braak score. The extensive overlap of soluble Aβ levels in controls with AD contrasts with the PET findings on fibrillar Aβ. These findings further support fibrillar Aβ as a biomarker for AD treatments and show the need for more detailed postmortem analysis of diverse soluble and insoluble Aβ aggregates in relation to PET.

Microglial P2X4 receptors promote ApoE degradation and contribute to memory deficits in Alzheimer's disease

Numerous evidences support that microglia contributes to the progression of Alzheimer's disease. P2X4 receptors are ATP-gated channels with high calcium permeability, which are de novo expressed in a subset of reactive microglia associated with various pathological contexts, contributing to microglial functions. P2X4 receptors are mainly localized in lysosomes and trafficking to the plasma membrane is tightly regulated. Here, we investigated the role of P2X4 in the context of Alzheimer's disease (AD). Using proteomics, we identified Apolipoprotein E (ApoE) as a specific P2X4 interacting protein. We found that P2X4 regulates lysosomal cathepsin B (CatB) activity promoting ApoE degradation; P2rX4 deletion results in higher amounts of intracellular and secreted ApoE in both bone-marrow-derived macrophage (BMDM) and microglia from APPswe/PSEN1dE9 brain. In both human AD brain and APP/PS1 mice, P2X4 and ApoE are almost exclusively expressed in plaque-associated microglia. In 12-month-old APP/PS1 mice, genetic deletion of P2rX4 reverses topographical and spatial memory impairment and reduces amount of soluble small aggregates of Aß1-42 peptide, while no obvious alteration of plaque-associated microglia characteristics is observed. Our results support that microglial P2X4 promotes lysosomal ApoE degradation, indirectly altering Aß peptide clearance, which in turn might promotes synaptic dysfunctions and cognitive deficits. Our findings uncover a specific interplay between purinergic signaling, microglial ApoE, soluble Aß (sAß) species and cognitive deficits associated with AD.

The Multifaceted Role of WNT Signaling in Alzheimer's Disease Onset and Age-Related Progression

The evolutionary conserved WNT signaling pathway orchestrates numerous complex biological processes during development and is critical to the maintenance of tissue integrity and homeostasis in the adult. As it relates to the central nervous system, WNT signaling plays several roles as it relates to neurogenesis, synaptic formation, memory, and learning. Thus, dysfunction of this pathway is associated with multiple diseases and disorders, including several neurodegenerative disorders. Alzheimer's disease (AD) is characterized by several pathologies, synaptic dysfunction, and cognitive decline. In this review, we will discuss the various epidemiological, clinical, and animal studies that demonstrate a precise link between aberrant WNT signaling and AD-associated pathologies. In turn, we will discuss the manner in which WNT signaling influences multiple molecular, biochemical, and cellular pathways upstream of these end-point pathologies. Finally, we will discuss how merging tools and technologies can be used to generate next generation cellular models to dissect the relationship between WNT signaling and AD.

A novel regulator in Alzheimer's disease progression: The astrocyte-derived extracellular vesicles

Alzheimer's disease (AD) is known as an age-related irreversible neurodegenerative disease. AD seriously endangers the health of the elderly, but there is still no effective treatment. In the past several decades, the significant role of astrocytes in the process of AD has been universally acknowledged. In addition, extracellular vesicles (EVs) have been recognized as an essential mediator in intercellular communication and participate in various pathophysiological processes by carrying and transporting diverse cargoes. Moreover, specific conditions and stimuli can modulate the amount and properties of astrocyte-derived EVs (ADEVs) to affect AD progression. Thus, recent studies focused on the involvement of ADEVs in the pathogenesis of AD and the potential application of ADEVs in the diagnosis and treatment of AD, which provides a new direction and possibility for revealing the mystery of AD. Interestingly, it can be concluded that ADEVs have both pathogenic and protective effects in the process of AD through a comprehensive generalization. In this review, we aim to summarize the multi-faces of ADEVs effects on AD development, which can provide a novel strategy to investigate the underlying mechanism in AD. We also summarize the current ADEVs clinically relevant studies to raise the potential use of ADEVs in the discovery of novel biomarkers for diagnosis and therapeutic targets for AD.

Rab11A Depletion in Microglia-Derived Extracellular Vesicle Proteome upon Beta-Amyloid Treatment

Microglia, the macrophage-like glial cells, behave as sentinels against exogenous pathogens invading the neural tissue. Their commitment is not only confined to the defensive function, but they also perform balancing trophic activities such as neuronal postnatal development, remodeling and pruning of synapses. Likewise, microglia-derived extracellular vesicles (EVs) can play strategic roles in maintaining a healthy brain by modulating neuronal activity and by controlling neurite outgrowth as well as innate immune response. Nevertheless, strong evidence also points to their role in the development of neurodegenerative pathologies such as Alzheimer's disease (AD). Here, we explored EV protein content released by BV2 microglial cells in a resting state and after stimulation with beta-amyloid peptides (Aβ), mimicking conditions occurring in AD. In the resting BV2 cells, we extended the list of proteins present in mouse microglia EV cargo with respect to those reported in the Vesiclepedia exosome database while, in amyloid-triggered microglia, we highlighted a pronounced drop in EV protein content. Focusing on Rab11A, a key factor in the recycling routes of amyloid species, we observed a dramatic decrease of this protein in Aβ-treated microglia EV cargo with respect to the EVs from the untreated sample. This decrease might affect the delivery of Rab11A to neurons thus increasing the harmful amyloid burden in neuronal cells that eventually may lead to their death. We tentatively proposed that alterations observed in EVs derived from Aβ-treated microglia may represent molecular features that, among others, shape the disease-associated microglial phenotype, a recently proposed subset of microglial population, present in neurodegenerative pathologies.

Endothelial LRP1-ICD Accelerates Cognition-Associated Alpha-Synuclein Pathology and Neurodegeneration through PARP1 Activation in a Mouse Model of Parkinson's Disease

Parkinson's disease (PD) is characterized by progressive loss of dopaminergic neurons and accumulation of misfolded alpha-synuclein (αSyn) into Lewy bodies. In addition to motor impairment, PD commonly presents with cognitive impairment, a non-motor symptom with poor outcome. Cortical αSyn pathology correlates closely with vascular risk factors and vascular degeneration in cognitive impairment. However, how the brain microvasculature regulates αSyn pathology and neurodegeneration remains unclear. Here, we constructed a rapidly progressive PD model by injecting alpha-synuclein preformed fibrils (αSyn PFFs) into the cerebral cortex and striatum. Brain capillaries in mice with cognitive impairment showed a reduction in diameter and length after 6 months, along with string vessel formation. The intracellular domain of low-density lipoprotein receptor-related protein-1 (LRP1-ICD) was upregulated in brain microvascular endothelium. LRP1-ICD promoted αSyn PFF uptake and exacerbated endothelial damage and neuronal apoptosis. Then, we overexpressed LRP1-ICD in brain capillaries using an adeno-associated virus carrying an endothelial-specific promoter. Endothelial LRP1-ICD worsened αSyn PFF-induced vascular damage, αSyn pathology, or neuron death in the cortex and hippocampus, resulting in severe motor and cognitive impairment. LRP1-ICD increased the synthesis of poly(adenosine 5'-diphosphate-ribose) (PAR) in the presence of αSyn PFFs. Inhibition of PAR polymerase 1 (PARP1) prevented vascular-derived injury, as did loss of PARP1 in the endothelium, which was further implicated in endothelial cell proliferation and inflammation. Together, we demonstrate a novel vascular mechanism of cognitive impairment in PD. These findings support a role for endothelial LRP1-ICD/PARP1 in αSyn pathology and neurodegeneration, and provide evidence for vascular protection strategies in PD therapy.

Analysis of Cerebral Small Vessel Changes in AD Model Mice

Amyloid β (Aβ) peptide is deposited in the brains of sporadic Alzheimer's disease (AD) due to impaired vessel-dependent clearance. To understand the mechanisms, we investigated time-dependent cerebrovascular changes in AD model mice. Cerebrovascular and other pathological changes were analyzed in AD model mice (J20 strain) aging from 2 to 9 months by immunostaining. At 2 months, Aβ was only intraneuronal, whereas vessels were positive from 3 months in J20 mice. Compared to wild-type (WT), vessel density was increased at 2 months but decreased at 9 months in J20 mice, claudin-5 levels were decreased, and vascular endothelial growth factor (VEGF) levels were increased in the cortex and hippocampus of J20 mice brain at all time points. Albumin extravasation was evident from 3 months in J20 brains. Collagen 4 was increased at 2 and 3 months. Aquaporin 4 was spread beyond the vessels starting from 3 months in J20, which was restricted around the vessel in wild-type mice. In conclusion, the study showed that an early decrease in claudin-5 was associated with VEGF expression, indicating dysfunction of the blood-brain barrier. Decreased claudin-5 might cause the leakage of blood constituents into the parenchyma that alters astrocyte polarity and its functions.

Rab11 and Its Role in Neurodegenerative Diseases

Vesicles mediate the trafficking of membranes/proteins in the endocytic and secretory pathways. These pathways are regulated by small GTPases of the Rab family. Rab proteins belong to the Ras superfamily of GTPases, which are significantly involved in various intracellular trafficking and signaling processes in the nervous system. Rab11 is known to play a key role especially in recycling many proteins, including receptors important for signal transduction and preservation of functional activities of nerve cells. Rab11 activity is controlled by GEFs (guanine exchange factors) and GAPs (GTPase activating proteins), which regulate its function through modulating GTP/GDP exchange and the intrinsic GTPase activity, respectively. Rab11 is involved in the transport of several growth factor molecules important for the development and repair of neurons. Overexpression of Rab11 has been shown to significantly enhance vesicle trafficking. On the other hand, a reduced expression of Rab11 was observed in several neurodegenerative diseases. Current evidence appears to support the notion that Rab11 and its cognate proteins may be potential targets for therapeutic intervention. In this review, we briefly discuss the function of Rab11 and its related interaction partners in intracellular pathways that may be involved in neurodegenerative processes.

Imipramine and olanzapine block apoE4-catalyzed polymerization of Aβ and show evidence of improving Alzheimer’s disease cognition

Background

The apolipoprotein E (APOE) ε4 allele confers the strongest risk for late-onset Alzheimer’s disease (AD) besides age itself, but the mechanisms underlying this risk are debated. One hypothesis supported by evidence from multiple labs is that apoE4 binds to the amyloid-β (Aβ) peptide and catalyzes its polymerization into neurotoxic oligomers and fibrils. Inhibiting this early step in the amyloid cascade may thereby reduce or prevent neurodegeneration and AD.

Methods

Using a design of experiments (DOE) approach, we developed a high-throughput assay to identify inhibitors of apoE4-catalyzed polymerization of Aβ into oligomers and fibrils. We used it to screen the NIH Clinical Collection of small molecule drugs tested previously in human clinical trials. We then evaluated the efficacy and cytotoxicity of the hit compounds in primary neuron models of apoE4-induced Aβ and phosphorylated tau aggregation. Finally, we performed retrospective analyses of the National Alzheimer’s Coordinating Center (NACC) clinical dataset, using Cox regression and Cox proportional hazards models to determine if the use of two FDA-approved hit compounds was associated with better cognitive scores (Mini-Mental State Exam), or improved AD clinical diagnosis, when compared with other medications of the same clinical indication.

Results

Our high-throughput screen identified eight blood-brain barrier (BBB)-permeable hit compounds that reduced apoE4-catalyzed Aβ oligomer and fibril formation in a dose-dependent manner. Five hit compounds were non-toxic toward cultured neurons and also reduced apoE4-promoted Aβ and tau neuropathology in a dose-dependent manner. Three of the five compounds were determined to be specific inhibitors of apoE4, whereas the other two compounds were Aβ or tau aggregation inhibitors. When prescribed to AD patients for their normal clinical indications, two of the apoE4 inhibitors, imipramine and olanzapine, but not other (non-hit) antipsychotic or antidepressant medications, were associated with improvements in cognition and clinical diagnosis, especially among APOE4 carriers.

Conclusions

The critical test of any proposed AD mechanism is whether it leads to effective treatments. Our high-throughput screen identified two promising FDA-approved drugs, imipramine and olanzapine, which have no structural, functional, or clinical similarities other than their shared ability to inhibit apoE4-catalyzed Aβ polymerization, thus identifying this mechanism as an essential contribution of apoE4 to AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-022-01020-9.

APOE ε2 resilience for Alzheimer's disease is mediated by plasma lipid species: Analysis of three independent cohort studies

INTRODUCTION

The apolipoprotein E (APOE) genotype is the strongest genetic risk factor for late-onset Alzheimer's disease. However, its effect on lipid metabolic pathways, and their mediating effect on disease risk, is poorly understood.

METHODS

We performed lipidomic analysis on three independent cohorts (the Australian Imaging, Biomarkers and Lifestyle [AIBL] flagship study, n = 1087; the Alzheimer's Disease Neuroimaging Initiative [ADNI] 1 study, n = 819; and the Busselton Health Study [BHS], n = 4384), and we defined associations between APOE ε2 and ε4 and 569 plasma/serum lipid species. Mediation analysis defined the proportion of the treatment effect of the APOE genotype mediated by plasma/serum lipid species.

RESULTS

A total of 237 and 104 lipid species were associated with APOE ε2 and ε4, respectively. Of these 68 (ε2) and 24 (ε4) were associated with prevalent Alzheimer's disease. Individual lipid species or lipidomic models of APOE genotypes mediated up to 30% and 10% of APOE ε2 and ε4 treatment effect, respectively.

DISCUSSION

Plasma lipid species mediate the treatment effect of APOE genotypes on Alzheimer's disease and as such represent a potential therapeutic target.

APOE in the bullseye of neurodegenerative diseases: impact of the APOE genotype in Alzheimer’s disease pathology and brain diseases

ApoE is the major lipid and cholesterol carrier in the CNS. There are three major human polymorphisms, apoE2, apoE3, and apoE4, and the genetic expression of APOE4 is one of the most influential risk factors for the development of late-onset Alzheimer's disease (AD). Neuroinflammation has become the third hallmark of AD, together with Amyloid-β plaques and neurofibrillary tangles of hyperphosphorylated aggregated tau protein. This review aims to broadly and extensively describe the differential aspects concerning apoE. Starting from the evolution of apoE to how APOE's single-nucleotide polymorphisms affect its structure, function, and involvement during health and disease. This review reflects on how APOE's polymorphisms impact critical aspects of AD pathology, such as the neuroinflammatory response, particularly the effect of APOE on astrocytic and microglial function and microglial dynamics, synaptic function, amyloid-β load, tau pathology, autophagy, and cell–cell communication. We discuss influential factors affecting AD pathology combined with the APOE genotype, such as sex, age, diet, physical exercise, current therapies and clinical trials in the AD field. The impact of the APOE genotype in other neurodegenerative diseases characterized by overt inflammation, e.g., alpha- synucleinopathies and Parkinson's disease, traumatic brain injury, stroke, amyotrophic lateral sclerosis, and multiple sclerosis, is also addressed. Therefore, this review gathers the most relevant findings related to the APOE genotype up to date and its implications on AD and CNS pathologies to provide a deeper understanding of the knowledge in the APOE field.

Therapeutic Targeting of Rab GTPases: Relevance for Alzheimer’s Disease

Rab GTPases (Rabs) are small proteins that play crucial roles in vesicle transport and membrane trafficking. Owing to their widespread functions in several steps of vesicle trafficking, Rabs have been implicated in the pathogenesis of several disorders, including cancer, diabetes, and multiple neurodegenerative diseases. As treatments for neurodegenerative conditions are currently rather limited, the identification and validation of novel therapeutic targets, such as Rabs, is of great importance. This review summarises proof-of-concept studies, demonstrating that modulation of Rab GTPases in the context of Alzheimer’s disease (AD) can ameliorate disease-related phenotypes, and provides an overview of the current state of the art for the pharmacological targeting of Rabs. Finally, we also discuss the barriers and challenges of therapeutically targeting these small proteins in humans, especially in the context of AD.

Long-term running exercise alleviates cognitive dysfunction in APP/PSEN1 transgenic mice via enhancing brain lysosomal function

Amyloid-β peptide (Aβ) aggregation is the hallmark of Alzheimer's disease (AD). The imbalance between the production and clearance of Aβ results in the accumulation and aggregation of Aβ in the brain. Thus far, few drugs are available for AD treatment, but exercise has been recognized for its cognition-enhancing properties in AD patients. The underlying mechanisms remain unclear. Our recent study showed that long-term running exercise could activate the lysosomal function in the brains of mice. In this study, we investigated whether exercise could reduce Aβ accumulation by activating lysosomal function in APP/PSEN1 transgenic mice. Started at the age of 5 months, the mice were trained with a running wheel at the speed of 18 r/min, 40 min/d, 6 d/week for 5 months, and were killed at the end of the 10th month, then brain tissue was collected for biochemical analyses. The cognitive ability was assessed in the 9th month. We showed that long-term exercise significantly mitigated cognitive dysfunction in AD mice, accompanied by the enhanced lysosomal function and the clearance of Aβ in the brain. Exercise significantly promoted the nuclear translocation of transcription factor EB (TFEB), and increased the interaction between nuclear TFEB with AMPK-mediated acetyl-CoA synthetase 2, thus enhancing transcription of the genes associated with the biogenesis of lysosomes. Exercise also raised the levels of mature cathepsin D and cathepsin L, suggesting that more Aβ peptides could be degraded in the activated lysosomes. This study demonstrates that exercise may improve the cognitive dysfunction of AD by enhancing lysosomal function.

Mechanistic insight into the role of metformin in Alzheimer's disease

Alzheimer's disease (AD), a type of dementia, is characterized by progressive memory decline and cognition impairment. Despite the considerable body of evidence regarding AD pathophysiology, current therapies merely slow down the disease progression, and a comprehensive therapeutic approach is unavailable. Accordingly, finding an efficient multifunctional remedy is necessary to blunt the increasing rate of AD incidence in the upcoming years. AD shares pathophysiological similarities (e.g., impairment of cognitive functions, insulin sensitivity, and brain glucose metabolism) with noninsulin-dependent diabetes mellitus (NIDDM), which offers the utilization of metformin, a biguanide hypoglycemic agent, as an alternative therapeutic approach in AD therapy. Emerging evidence has revealed the impact of metformin in patients suffering from AD. It has been described that metformin employs multiple mechanisms to improve cognition and memory impairment in pre-clinical AD models, including reduction of hippocampal amyloid-beta (Aβ) plaque and neurofibrillary tangles (NFTs) load, suppression of inflammation, amelioration of mitochondrial dysfunction and oxidative stress, restriction of apoptotic neuronal death, and induction of neurogenesis. This review discusses the pre-clinical evidence, which may shed light on the role of metformin in AD and provide a more comprehensive mechanistic insight for future studies in this area of research.

Extracellular vesicles released by human retinal pigment epithelium mediate increased polarised secretion of drusen proteins in response to AMD stressors

Age-related macular degeneration (AMD) is a leading cause of blindness worldwide. Drusen are key contributors to the etiology of AMD and the ability to modulate drusen biogenesis could lead to therapeutic strategies to slow or halt AMD progression. The mechanisms underlying drusen biogenesis, however, remain mostly unknown. Here we demonstrate that under homeostatic conditions extracellular vesicles (EVs) secreted by retinal pigment epithelium (RPE) cells are enriched in proteins associated with mechanisms involved in AMD pathophysiology, including oxidative stress, immune response, inflammation, complement system and drusen composition. Furthermore, we provide first evidence that drusen-associated proteins are released as cargo of extracellular vesicles secreted by RPE cells in a polarised apical:basal mode. Notably, drusen-associated proteins exhibited distinctive directional secretion modes in homeostatic conditions and, differential modulation of this directional secretion in response to AMD stressors. These observations underpin the existence of a finely-tuned mechanism regulating directional apical:basal sorting and secretion of drusen-associated proteins via EVs, and its modulation in response to mechanisms involved in AMD pathophysiology. Collectively, our results strongly support an active role of RPE-derived EVs as a key source of drusen proteins and important contributors to drusen development and growth.

Abnormal amyloid beta metabolism in systemic abnormalities and Alzheimer's pathology: Insights and therapeutic approaches from periphery

Alzheimer's disease (AD) is an age-associated, multifactorial neurodegenerative disorder that is incurable. Despite recent success in treatments that partially improve symptomatic relief, they have failed in most clinical trials. Re-holding AD for accurate diagnosis and treatment is widely known as a challenging task. Lack of knowledge of basic molecular pathogenesis might be a possible reason for ineffective AD treatment. Historically, a majority of therapy-based studies have investigated the role of amyloid-β (Aβ peptide) in the central nervous system (CNS), whereas less is known about Aβ peptide in the periphery in AD. In this review, we provide a comprehensive summary of the current understanding of Aβ peptide metabolism (anabolism and catabolism) in the brain and periphery. We show that the abnormal metabolism of Aβ peptide is significantly linked with central-brain and peripheral abnormalities; the interaction between peripheral Aβ peptide metabolism and peripheral abnormalities affects central-brain Aβ peptide metabolism, suggesting the existence of significant communication between these two pathways of Aβ peptide metabolism. This close interaction between the central brain and periphery in abnormal Aβ peptide metabolism plays a key role in the development and progression of AD. In conclusion, we need to obtain a full understanding of the dynamic roles of Aβ peptide at the molecular level in both the brain and periphery in relation to the pathology of AD. This will not only provide new information regarding the complex disease pathology, but also offer potential new clues to improve therapeutic strategies and diagnostic biomarkers for the successful treatment of AD.

Apolipoprotein E isoform-dependent effects on the processing of Alzheimer's amyloid-β

Since the identification of the apolipoprotein E (apoE) *ε4 allele as a major genetic risk factor for late-onset Alzheimer's disease, significant efforts have been aimed at elucidating how apoE4 expression confers greater brain amyloid-β (Aβ) burden, earlier disease onset and worse clinical outcomes compared to apoE2 and apoE3. ApoE primarily functions as a lipid carrier to regulate cholesterol metabolism in circulation as well as in the brain. However, it has also been suggested to interact with hydrophobic Aβ peptides to influence their processing in an isoform-dependent manner. Here, we review evidence from in vitro and in vivo studies extricating the effects of the three apoE isoforms, on different stages of the Aβ processing pathway including synthesis, aggregation, deposition, clearance and degradation. ApoE4 consistently correlates with impaired Aβ clearance, however data regarding Aβ synthesis and aggregation are conflicting and likely reflect inconsistencies in experimental approaches across studies. We further discuss the physical and chemical properties of apoE that may explain the inherent differences in activity between the isoforms. The lipidation status and lipid transport function of apoE are intrinsically linked with its ability to interact with Aβ. Traditionally, apoE-oriented therapeutic strategies for Alzheimer's disease have been proposed to non-specifically enhance or inhibit apoE activity. However, given the wide-ranging physiological functions of apoE in the brain and periphery, a more viable approach may be to specifically target and neutralise the pathological apoE4 isoform.

Apolipoprotein E and Alzheimer's Disease: Findings, Hypotheses, and Potential Mechanisms

Alzheimer's disease (AD) is a multifactorial neurodegenerative disorder that involves dysregulation of many cellular and molecular processes. It is notoriously difficult to develop therapeutics for AD due to its complex nature. Nevertheless, recent advancements in imaging technology and the development of innovative experimental techniques have allowed researchers to perform in-depth analyses to uncover the pathogenic mechanisms of AD. An important consideration when studying late-onset AD is its major genetic risk factor, apolipoprotein E4 (apoE4). Although the exact mechanisms underlying apoE4 effects on AD initiation and progression are not fully understood, recent studies have revealed critical insights into the apoE4-induced deficits that occur in AD. In this review, we highlight notable studies that detail apoE4 effects on prominent AD pathologies, including amyloid-β, tau pathology, neuroinflammation, and neural network dysfunction. We also discuss evidence that defines the physiological functions of apoE and outlines how these functions are disrupted in apoE4-related AD. Expected final online publication date for the Annual Review of Pathology: Mechanisms of Disease, Volume 17 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Opposing functions for retromer and Rab11 in extracellular vesicle traffic at presynaptic terminals

Neuronal extracellular vesicles (EVs) play important roles in intercellular communication and pathogenic protein propagation in neurological disease. However, it remains unclear how cargoes are selectively packaged into neuronal EVs. Here, we show that loss of the endosomal retromer complex leads to accumulation of EV cargoes including amyloid precursor protein (APP), synaptotagmin-4 (Syt4), and neuroglian (Nrg) at Drosophila motor neuron presynaptic terminals, resulting in increased release of these cargoes in EVs. By systematically exploring known retromer-dependent trafficking mechanisms, we show that EV regulation is separable from several previously identified roles of neuronal retromer. Conversely, mutations in rab11 and rab4, regulators of endosome-plasma membrane recycling, cause reduced EV cargo levels, and rab11 suppresses cargo accumulation in retromer mutants. Thus, EV traffic reflects a balance between Rab4/Rab11 recycling and retromer-dependent removal from EV precursor compartments. Our data shed light on previous studies implicating Rab11 and retromer in competing pathways in Alzheimer's disease, and suggest that misregulated EV traffic may be an underlying defect.

Role of Microglia in Regulating Cholesterol and Tau Pathology in Alzheimer's Disease

Cholesterol, a principal constituent of the cell membrane, plays a crucial role in the brain by regulating the synaptic transmission, neuronal signaling, as well as neurodegenerative diseases. Defects in the cholesterol trafficking are associated with enhanced generation of hyperphosphorylated Tau and Amyloid-β protein. Tau, a major microtubule-associated protein in the brain, is the key regulator of the mature neuron. Abnormally hyperphosphorylated Tau hampers the major functions related to microtubule assembly by promoting neurofibrillary tangles of paired helical filaments, twisted ribbons, and straight filaments. The observed pathological changes due to impaired cholesterol and Tau protein accumulation cause Alzheimer's disease. Thus, in order to regulate the pathogenesis of Alzheimer's disease, regulation of cholesterol metabolism, as well as Tau phosphorylation, is essential. The current review provides an overview of (1) cholesterol synthesis in the brain, neurons, astrocytes, and microglia; (2) the mechanism involved in modulating cholesterol concentration between the astrocytes and brain; (3) major mechanisms involved in the hyperphosphorylation of Tau and amyloid-β protein; and (4) microglial involvement in its regulation. Thus, the answering key questions will provide an in-depth information on microglia involvement in managing the pathogenesis of cholesterol-modulated hyperphosphorylated Tau protein.

Small GTPases of the Rab and Arf Families: Key Regulators of Intracellular Trafficking in Neurodegeneration

Small guanosine triphosphatases (GTPases) of the Rab and Arf families are key regulators of vesicle formation and membrane trafficking. Membrane transport plays an important role in the central nervous system. In this regard, neurons require a constant flow of membranes for the correct distribution of receptors, for the precise composition of proteins and organelles in dendrites and axons, for the continuous exocytosis/endocytosis of synaptic vesicles and for the elimination of dysfunctional proteins. Thus, it is not surprising that Rab and Arf GTPases have been associated with neurodegenerative diseases such as Alzheimer’s and Parkinson’s. Both pathologies share characteristics such as the presence of protein aggregates and/or the fragmentation of the Golgi apparatus, hallmarks that have been related to both Rab and Arf GTPases functions. Despite their relationship with neurodegenerative disorders, very few studies have focused on the role of these GTPases in the pathogenesis of neurodegeneration. In this review, we summarize their importance in the onset and progression of Alzheimer’s and Parkinson’s diseases, as well as their emergence as potential therapeutical targets for neurodegeneration.

Degradation Products of Amyloid Protein: Are They The Culprits?

Objectives:

Beta-amyloid (Aβ) peptides are most toxic to cells in oligomeric form. It is commonly accepted that oligomers can form ion channels in cell membranes and allow calcium and other ions to enter cells. The activation of other mechanisms, such as apoptosis or lipid peroxidation, aggravates the toxicity, but it itself can result from the same initial point, that is, ion disturbance due to an increased permeability of membranes. However, experimental studies of membrane channels created by Aβ are surprisingly limited. Methods: Here, we report a novel flow cytometry technique which can be used to detect increased permeability of membranes to calcium induced by the exposure to amyloid peptides. Calcium entry into the liposome is monitored using calcium-sensitive fluorescent probe. Undamaged lipid membranes are not permeable to calcium. Liposomes that are prepared in a calcium-free medium become able to accumulate calcium in a calcium-containing medium only after the formation of channels. Using this technique, we demonstrated that the addition of short amyloid fragment Aβ, which is known for its extreme toxicity on cultured neurons, readily increased membrane permeability to calcium. However, neither similarly sized peptide Ab22-35 nor full-length peptide Ab1-42 were producing channels. The formation of channels was observed in the membranes made of phosphatidylserine, a negatively charged lipid, but not in membranes made of the neutral phosphatidylcholine.

Methods:

Here, we report a novel flow cytometry technique which can be used to detect increased permeability of membranes to calcium induced by the exposure to amyloid peptides. Calcium entry into the liposome is monitored using calcium-sensitive fluorescent probe. Undamaged lipid membranes are not permeable to calcium. Liposomes that are prepared in a calcium-free medium become able to accumulate calcium in a calcium-containing medium only after the formation of channels. Using this technique, we demonstrated that the addition of short amyloid fragment Aβ, which is known for its extreme toxicity on cultured neurons, readily increased membrane permeability to calcium. However, neither similarly sized peptide Ab22-35 nor full-length peptide Ab1-42 were producing channels. The formation of channels was observed in the membranes made of phosphatidylserine, a negatively charged lipid, but not in membranes made of the neutral phosphatidylcholine.

Results:

In the Discussion section, we have analyzed several issues which could be critical for understanding the pathogenesis of Alzheimer’s disease, specifically 1) the need for a negatively charged membrane to produce the ion channel; 2) the potential role of the aggregated form in cellular toxicity of Ab peptides; 3) channel-forming ability of multiple degradation products of amyloid; 4) non-specificity of ion channels formed by amyloid peptides. Potential targets of channel-forming oligomers appear to be intracellular and are organelles well-known for dysfunction in Alzheimer’s disease (mitochondria and lysosomes). In fact, lysosomes can also be the producers of degraded amyloid. Provided speculations support the hypothesis that neuronal toxicity can be caused by the degradation products of beta-amyloid.

Cathepsin D: A Candidate Link between Amyloid β-protein and Tauopathy in Alzheimer Disease

Alzheimer disease (AD) is a debilitating neurodegenerative disorder characterized by extracellular deposition of the amyloid β-protein (Aβ) and intraneuronal accumulation of the microtubule-associated protein, tau. Despite a wealth of experimental and genetic evidence implicating both Aβ and tau in the pathogenesis of AD, the precise molecular links between these two pathological hallmarks have remained surprisingly elusive. Here, we review emerging evidence for a critical nexus among Aβ, tau, and the lysosomal protease cathepsin D (CatD) that we hypothesize may play a pivotal role in the etiology of AD. CatD degrades both Aβ and tau in vitro, but the in vivo relevance of this lysosomal protease to these principally extracellular and cytosolic proteins, respectively, had remained undefined for many decades. Recently, however, our group found that genetic deletion of CatD in mice results in dramatic accumulation of Aβ in lysosomes, revealing that Aβ is normally trafficked to lysosomes in substantial quantities. Moreover, emerging evidence suggests that tau is also trafficked to the lysosome via chaperone-mediated autophagy and other trafficking pathways. Thus, Aβ, tau and CatD are colocalized in the lysosome, an organelle that shows dysfunction early in AD pathogenesis, where they can potentially interact. Notably, we discovered that Aβ42-the Aβ species most strongly linked to AD pathogenesis-is a highly potent, low-nanomolar, competitive inhibitor of CatD. Taking these observations together, we hypothesize that Aβ42 may trigger tauopathy by competitive inhibition of CatD-mediated degradation of tau-pathogenic forms of tau, in particular. Herein, we review the evidence supporting this hypothesis and explore the implications for the molecular pathogenesis of AD. Future research into these novel mechanistic links among Aβ, tau and CatD promises to expand our understanding of the etiology of AD and could potentially lead to novel therapeutic approaches for combatting this devastating disease of brain and mind.

APOE4 is associated with cognitive and pathological heterogeneity in patients with Alzheimer's disease: a systematic review

Possession of the ε4 allele of apolipoprotein E (APOE) is the primary genetic risk factor for the sporadic form of Alzheimer’s disease (AD). While researchers have extensively characterized the impact that APOE ε4 (APOE4) has on the susceptibility of AD, far fewer studies have investigated the phenotypic differences of patients with AD who are APOE4 carriers vs. those who are non-carriers. In order to understand these differences, we performed a qualitative systematic literature review of the reported cognitive and pathological differences between APOE4-positive (APOE4+) vs. APOE4-negative (APOE4−) AD patients. The studies performed on this topic to date suggest that APOE4 is not only an important mediator of AD susceptibility, but that it likely confers specific phenotypic heterogeneity in AD presentation, as well. Specifically, APOE4+ AD patients appear to possess more tau accumulation and brain atrophy in the medial temporal lobe, resulting in greater memory impairment, compared to APOE4− AD patients. On the other hand, APOE4− AD patients appear to possess more tau accumulation and brain atrophy in the frontal and parietal lobes, resulting in greater impairment in executive function, visuospatial abilities, and language, compared to APOE4+ AD patients. Although more work is necessary to validate and interrogate these findings, these initial observations of pathological and cognitive heterogeneity between APOE4+ vs. APOE4− AD patients suggest that there is a fundamental divergence in AD manifestation related to APOE genotype, which may have important implications in regard to the therapeutic treatment of these two patient populations.

Endocytosis Is a Key Mode of Interaction between Extracellular β-Amyloid and the Cell Membrane

Interactions between amyloid-β peptide (Aβ) and the cell membrane include interaction with membrane lipids and binding to membrane receptors, both of which are considered to be the toxicity mechanisms of Aβ. However, it is unclear whether both mechanisms lead to cytotoxicity. Thus, we aimed to analyze these two mechanisms of Aβ42 interaction with cell membranes under different Aβ aggregation states. To this end, model membrane experiments were conducted. Quantitative analysis of Aβ42 monomers or oligomers bound to the membrane of neuro-2a cells was also performed, and laser confocal microscopy was employed to assess endocytosis of FITC-Aβ42 monomers or oligomers by neuro-2a cells. We found that the binding capacity of Aβ42 to membrane lipids was weak and that the amount of Aβ42 bound to membrane lipids was low. Moreover, clathrin-mediated endocytosis of Aβ42 oligomers by neuro-2a cells was observed. Endocytosis serves as a key mode of interaction between extracellular Aβ42 and neurons. These findings provide insights into the mechanisms underlying Aβ oligomer metabolism.

Vps54 regulates Drosophila neuromuscular junction development and interacts genetically with Rab7 to control composition of the postsynaptic density

Vps54 is a subunit of the Golgi-associated retrograde protein (GARP) complex, which is involved in tethering endosome-derived vesicles to the trans-Golgi network (TGN). In the wobbler mouse, a model for human motor neuron (MN) disease, reduction in the levels of Vps54 causes neurodegeneration. However, it is unclear how disruption of the GARP complex leads to MN dysfunction. To better understand the role of Vps54 in MNs, we have disrupted expression of the Vps54 ortholog in Drosophila and examined the impact on the larval neuromuscular junction (NMJ). Surprisingly, we show that both null mutants and MN-specific knockdown of Vps54 leads to NMJ overgrowth. Reduction of Vps54 partially disrupts localization of the t-SNARE, Syntaxin-16, to the TGN but has no visible impact on endosomal pools. MN-specific knockdown of Vps54 in MNs combined with overexpression of the small GTPases Rab5, Rab7, or Rab11 suppresses the Vps54 NMJ phenotype. Conversely, knockdown of Vps54 combined with overexpression of dominant negative Rab7 causes NMJ and behavioral abnormalities including a decrease in postsynaptic Dlg and GluRIIB levels without any effect on GluRIIA. Taken together, these data suggest that Vps54 controls larval MN axon development and postsynaptic density composition through a mechanism that requires Rab7.

Rab11-mediated recycling endosome role in nervous system development and neurodegenerative diseases

STUDY

Membrane trafficking process is significant for the complex and precise regulatory of the nervous system. Rab11, as a small GTPase of the Rab superfamily, controls endocytic vesicular trafficking to the cell surface after sorting in recycling endosome (RE), coordinating with its receptors to maintain neurological function.

MATERIALS AND METHODS

This article reviewed the literature of Rab11 in nervous system.

RESULTS

Rab11-positive vesicles targeted transport growth-related molecules, such as integrins, protrudin, tropomyosin receptor kinase (Trk) A/B receptor and AMPA receptor (AMPAR) to membrane surface to promote the regeneration capacity of axon and dendrites and maintain synaptic plasticity. In addition, many studies have shown that the expression of Rab11 is decreased in multiple neurodegenerative diseases, which is highly correlated with the process of production, transport and clearance of disease-related pathological proteins. And overexpression or increased activity of Rab11 can greatly improve the defect of membrane trafficking and slow down the disease process.

CONCLUSION

With increasing research efforts on Rab11-dependent membrane trafficking mechanisms, a potential target for nerve regeneration and neurodegenerative diseases will be provided.

Autophagic and endo-lysosomal dysfunction in neurodegenerative disease

Due to their post-mitotic state, metabolic demands and often large polarised morphology, the function and survival of neurons is dependent on an efficient cellular waste clearance system both for generation of materials for metabolic processes and removal of toxic components. It is not surprising therefore that deficits in protein clearance can tip the balance between neuronal health and death. Here we discuss how autophagy and lysosome-mediated degradation pathways are disrupted in several neurological disorders. Both genetic and cell biological evidence show the diversity and complexity of vesicular clearance dysregulation in cells, and together may ultimately suggest a unified mechanism for neuronal demise in degenerative conditions. Causative and risk-associated mutations in Alzheimer's disease, Frontotemporal Dementia, Amyotrophic Lateral Sclerosis, Parkinson's disease, Huntington's disease and others have given the field a unique mechanistic insight into protein clearance processes in neurons. Through their broad implication in neurodegenerative diseases, molecules involved in these genetic pathways, in particular those involved in autophagy, are emerging as appealing therapeutic targets for intervention in neurodegeneration.

Rab GTPases and Membrane Trafficking in Neurodegeneration

Defects in membrane trafficking are hallmarks of neurodegeneration. Rab GTPases are key regulators of membrane trafficking. Alterations of Rab GTPases, or the membrane compartments they regulate, are associated with virtually all neuronal activities in health and disease. The observation that many Rab GTPases are associated with neurodegeneration has proven a challenge in the quest for cause and effect. Neurodegeneration can be a direct consequence of a defect in membrane trafficking. Alternatively, changes in membrane trafficking may be secondary consequences or cellular responses. The secondary consequences and cellular responses, in turn, may protect, represent inconsequential correlates or function as drivers of pathology. Here, we attempt to disentangle the different roles of membrane trafficking in neurodegeneration by focusing on selected associations with Alzheimer’s disease, Parkinson’s disease, Huntington’s disease and selected neuropathies. We provide an overview of current knowledge on Rab GTPase functions in neurons and review the associations of Rab GTPases with neurodegeneration with respect to the following classifications: primary cause, secondary cause driving pathology or secondary correlate. This analysis is devised to aid the interpretation of frequently observed membrane trafficking defects in neurodegeneration and facilitate the identification of true causes of pathology.

The Ras Superfamily of Small GTPases in Non-neoplastic Cerebral Diseases

The small GTPases from the Ras superfamily play crucial roles in basic cellular processes during practically the entire process of neurodevelopment, including neurogenesis, differentiation, gene expression, membrane and protein traffic, vesicular trafficking, and synaptic plasticity. Small GTPases are key signal transducing enzymes that link extracellular cues to the neuronal responses required for the construction of neuronal networks, as well as for synaptic function and plasticity. Different subfamilies of small GTPases have been linked to a number of non-neoplastic cerebral diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), intellectual disability, epilepsy, drug addiction, Huntington’s disease (HD), amyotrophic lateral sclerosis (ALS) and a large number of idiopathic cerebral diseases. Here, we attempted to make a clearer illustration of the relationship between Ras superfamily GTPases and non-neoplastic cerebral diseases, as well as their roles in the neural system. In future studies, potential treatments for non-neoplastic cerebral diseases which are based on small GTPase related signaling pathways should be explored further. In this paper, we review all the available literature in support of this possibility.

Apolipoprotein E/Amyloid-β Complex Accumulates in Alzheimer Disease Cortical Synapses via Apolipoprotein E Receptors and Is Enhanced by APOE4

Apolipoprotein E (apoE) colocalizes with amyloid-β (Aβ) in Alzheimer disease (AD) plaques and in synapses, and evidence suggests that direct interactions between apoE and Aβ are important for apoE's effects in AD. The present work examines the hypothesis that apoE receptors mediate uptake of apoE/Aβ complex into synaptic terminals. Western blot analysis shows multiple SDS-stable assemblies in synaptosomes from human AD cortex; apoE/Aβ complex was markedly increased in AD compared with aged control samples. Complex formation between apoE and Aβ was confirmed by coimmunoprecipitation experiments. The apoE receptors low-density lipoprotein receptor (LDLR) and LDLR-related protein 1 (LRP1) were quantified in synaptosomes using flow cytometry, revealing up-regulation of LRP1 in early- and late-stage AD. Dual-labeling flow cytometry analysis of LRP1- and LDLR positives indicate most (approximately 65%) of LDLR and LRP1 is associated with postsynaptic density-95 (PSD-95)-positive synaptosomes, indicating that remaining LRP1 and LDLR receptors are exclusively presynaptic. Flow cytometry analysis of Nile red labeling revealed a reduction in cholesterol esters in AD synaptosomes. Dual-labeling experiments showed apoE and Aβ concentration into LDLR and LRP1-positive synaptosomes, along with free and esterified cholesterol. Synaptic Aβ was increased by apoE4 in control and AD samples. These results are consistent with uptake of apoE/Aβ complex and associated lipids into synaptic terminals, with subsequent Aβ clearance in control synapses and accumulation in AD synapses.

Ordered and Disordered Segments of Amyloid-β Drive Sequential Steps of the Toxic Pathway

While the roles of intrinsically disordered protein domains in driving interprotein interactions are increasingly well-appreciated, the mechanism of toxicity of disease-causing disordered proteins remains poorly understood. A prime example is Alzheimer's disease (AD) associated amyloid beta (Aβ). Aβ oligomers are highly toxic partially structured peptide assemblies with a distinct ordered region (residues ∼10-40) and a shorter disordered region (residues ∼1-9). Here, we investigate the role of this disordered domain and its relation to the ordered domain in the manifestation of toxicity through a set of Aβ fragments and stereoisomers designed for this purpose. We measure their effects on lipid membranes and cultured neurons, probing their toxicity, intracellular distributions, and specific molecular interactions using the techniques of confocal imaging, lattice light sheet imaging, fluorescence lifetime imaging, and fluorescence correlation spectroscopy. Remarkably, we find that neither part-Aβ_10-40 or Aβ_1-9, is toxic by itself. The ordered part (Aβ_10-40) is the major determinant of how Aβ attaches to lipid bilayers, enters neuronal cells, and localizes primarily in the late endosomal compartments. However, once Aβ enters the cell, it is the disordered part (only when it is connected to the rest of the peptide) that has a strong and stereospecific interaction with an unknown cellular component, as demonstrated by distinct changes in the fluorescence lifetime of a fluorophore attached to the N-terminal. This interaction appears to commit Aβ to the toxic pathway. Our findings correlate well with Aβ sites of familial AD mutations, a significant fraction of which cluster in the disordered region. We conclude that, while the ordered region dictates attachment and cellular entry, the key to toxicity lies in the ordered part presenting the disordered part for a specific cellular interaction.

Distinct functional roles of Vps41-mediated neuroprotection in Alzheimer's and Parkinson's disease models of neurodegeneration

Commonalities and, in some cases, pathological overlap between neurodegenerative diseases have led to speculation that targeting of underlying mechanisms might be of potentially shared therapeutic benefit. Alzheimer's disease is characterized by the formation of plaques, composed primarily of the amyloid-β 1-42 (Aβ) peptide in the brain, resulting in neurodegeneration. Previously, we have shown that overexpression of the lysosomal-trafficking protein, human Vps41 (hVps41), is neuroprotective in a transgenic worm model of Parkinson's disease, wherein progressive dopaminergic neurodegeneration is induced by α-synuclein overexpression. Here, we report the results of a systematic comparison of hVps41-mediated neuroprotection between α-synuclein and Aβ in transgenic nematode models of Caenorhabditis elegans. Our results indicate that an ARF-like GTPase gene product, ARL-8, mitigates endocytic Aβ neurodegeneration in a VPS-41-dependent manner, rather than through RAB-7 and AP3 as with α-synuclein. Furthermore, the neuroprotective effect of ARL-8 or hVps41 appears to be dependent on their colocalization and the activity of ARL-8. Additionally, we demonstrate that the LC3 orthologue, LGG-2, plays a critical role in Aβ toxicity with ARL-8. Further analysis of functional effectors of Aβ protein processing via the lysosomal pathway will assist in the elucidation of the underlying mechanism involving VPS-41-mediated neuroprotection. These results reveal functional distinctions in the intracellular management of neurotoxic proteins that serve to better inform the path for development of therapeutic interventions to halt neurodegeneration.

GBA and APOE ε4 associate with sporadic dementia with Lewy bodies in European genome wide association study

Dementia with Lewy Bodies (DLB) is a common neurodegenerative disorder with poor prognosis and mainly unknown pathophysiology. Heritability estimates exceed 30% but few genetic risk variants have been identified. Here we investigated common genetic variants associated with DLB in a large European multisite sample. We performed a genome wide association study in Norwegian and European cohorts of 720 DLB cases and 6490 controls and included 19 top-associated single-nucleotide polymorphisms in an additional cohort of 108 DLB cases and 75545 controls from Iceland. Overall the study included 828 DLB cases and 82035 controls. Variants in the ASH1L/GBA (Chr1q22) and APOE ε4 (Chr19) loci were associated with DLB surpassing the genome-wide significance threshold (p < 5 × 10-8). One additional genetic locus previously linked to psychosis in Alzheimer's disease, ZFPM1 (Chr16q24.2), showed suggestive association with DLB at p-value < 1 × 10-6. We report two susceptibility loci for DLB at genome-wide significance, providing insight into etiological factors. These findings highlight the complex relationship between the genetic architecture of DLB and other neurodegenerative disorders.

Phagocytosis of Apoptotic Cells Is Specifically Upregulated in ApoE4 Expressing Microglia in vitro

Alzheimer’s disease (AD) is characterized by intracellular tau aggregates and extracellular deposition of amyloid-β (Aβ). The major genetic risk factor to develop AD is the Apolipoprotein E isoform 4 (ApoE4). ApoE4 may directly affect Aβ pathology, yet the exact role of ApoE4 in the progression of AD remains unclear. Although astrocytes are the main source of ApoE in brain tissue, other cell types might contribute to ApoE isotype-dependent effects. While ApoE expression does not play a relevant role in homeostatic microglia, we and others could recently show that ApoE expression is significant upregulated in disease-associated microglia including AD-mouse models and human AD. ApoE has been supposed to have an anti-inflammatory effect, with ApoE4 being less effective than ApoE3. However, ApoE-isotype specific effects on microglia function in disease have not been thoroughly investigated to date. In contrast to this, the role of ApoE2, the third most common major ApoE isoform, in neurodegeneration has not been characterized in detail, but it has been shown to delay the onset of disease in familial AD. To elucidate the differential roles of the three-major human ApoE isoforms on microglia function we each expressed the human ApoE isoforms in murine N9 microglia cells. We could show that ApoE4 specifically influences actin cytoskeleton rearrangement and morphology. In migration assays, ApoE4 significantly promotes cell motility. To quantify phagocytosis by microglia we established an uptake assay based on imaging flow cytometry. Although expression of ApoE4 led to significantly reduced uptake of Aβ in contrast to the other isoforms, we could show that ApoE4 specifically increased phagocytosis of apoptotic neuronal cells. Our findings show that ApoE4 intrinsically affects microglia physiology by upregulating motility and phagocytic behavior in vitro and may therefore specifically contribute to microglia dysregulation in AD.

ApoE4: an emerging therapeutic target for Alzheimer's disease

BACKGROUND

The growing body of evidence indicating the heterogeneity of Alzheimer's disease (AD), coupled with disappointing clinical studies directed at a fit-for-all therapy, suggest that the development of a single magic cure suitable for all cases may not be possible. This calls for a shift in paradigm where targeted treatment is developed for specific AD subpopulations that share distinct genetic or pathological properties. Apolipoprotein E4 (apoE4), the most prevalent genetic risk factor of AD, is expressed in more than half of AD patients and is thus an important possible AD therapeutic target.

REVIEW

This review focuses initially on the pathological effects of apoE4 in AD, as well as on the corresponding cellular and animal models and the suggested cellular and molecular mechanisms which mediate them. The second part of the review focuses on recent apoE4-targeted (from the APOE gene to the apoE protein and its interactors) therapeutic approaches that have been developed in animal models and are ready to be translated to human. Further, the issue of whether the pathological effects of apoE4 are due to loss of protective function or due to gain of toxic function is discussed herein. It is possible that both mechanisms coexist, with certain constituents of the apoE4 molecule and/or its downstream signaling mediating a toxic effect, while others are associated with a loss of protective function.

CONCLUSION

ApoE4 is a promising AD therapeutic target that remains understudied. Recent studies are now paving the way for effective apoE4-directed AD treatment approaches.