Endosome function and dysfunction in Alzheimer's disease and other neurodegenerative diseases

Genes for endosomal pH regulators NHE6 and NHE9 are dysregulated in the substantia nigra in Parkinson's disease

Endosomal acid base balance functions as a master orchestrator within the cell, engaging with many cellular pathways to maintain homeostasis. Mutations in the endosomal pH regulator Na+/H+ exchanger NHE6 may disrupt this delicate balancing act and cause monogenic Parkinsonism. Here, gene expression studies in post-mortem substantia nigra of Parkinson's disease (PD) patients and normal controls were performed to investigate whether NHE6 represents a pathophysiological link between monogenic and sporadic PD. The substantia nigra in PD displayed down-regulation of NHE6, coincident with a loss of expression of several SNARE signalling pathway members, suggesting impaired membrane fusion and vesicle-recycling. Increased abundance of related NHE9 was also identified in the parkinsonian nigra that could reflect compensatory changes or be a consequence of neuronal dysfunction. The current model suggests the possibility that neurons expressing low levels of NHE6 are more susceptible to injury in PD, potentially directly contributing to the loss of nigral dopaminergic neurons and the genesis of the disease. These results have important implications for disease-modifying therapies as they suggest that endosomal pH correctors, including epigenetic modifiers that regulate NHE6 expression, may be beneficial for PD. Thus, aberrant endosomal acidification in the nigrostriatal pathway is a possible unifying pathomechanism in both monogenic and sporadic PD, with implications for understanding and treating this disorder. Replication of these observations in the post-mortem brains of Alzheimer's disease and frontotemporal dementia patients supports a model of conserved mechanisms underlying injury and death of neurons.

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.

Enhancing Targeted Drug Delivery through Cell-Specific Endosomal Escape

Endosome is a major barrier in the intracellular delivery of drugs, especially for biologics, such as proteins, peptides, and nucleic acids. After being endocytosed, these cargos will be trapped inside the endosomal compartments and finally degraded in the lysosomes. Thus, various strategies have been developed to facilitate the escape of cargos from the endosomes to improve the intracellular delivery efficiency. While the majority of the studies are focusing on strengthening the endosomal escape capability to maximize the delivery outcome, recent evidence suggests that a careful control of the endosomal escape process could provide opportunity for targeted drug delivery. In this concept review, we examined current delivery systems that can sense intra-endosomal factors or external stimuli for controlling endosomal escape toward a targeted intracellular delivery of cargos. Furthermore, the prospects and challenges of such strategies are discussed.

The Role of RIN3 Gene in Alzheimer's Disease Pathogenesis: a Comprehensive Review

Alzheimer's disease (AD) is a globally prevalent form of dementia that impacts diverse populations and is characterized by progressive neurodegeneration and impairments in executive memory. Although the exact mechanisms underlying AD pathogenesis remain unclear, it is commonly accepted that the aggregation of misfolded proteins, such as amyloid plaques and neurofibrillary tau tangles, plays a critical role. Additionally, AD is a multifactorial condition influenced by various genetic factors and can manifest as either early-onset AD (EOAD) or late-onset AD (LOAD), each associated with specific gene variants. One gene of particular interest in both EOAD and LOAD is RIN3, a guanine nucleotide exchange factor. This gene plays a multifaceted role in AD pathogenesis. Firstly, upregulation of RIN3 can result in endosomal enlargement and dysfunction, thereby facilitating the accumulation of beta-amyloid (Aβ) peptides in the brain. Secondly, RIN3 has been shown to impact the PICLAM pathway, affecting transcytosis across the blood-brain barrier. Lastly, RIN3 has implications for immune-mediated responses, notably through its influence on the PTK2B gene. This review aims to provide a concise overview of AD and delve into the role of the RIN3 gene in its pathogenesis.

Intracellular microbial rhodopsin-based optogenetics to control metabolism and cell signaling

Microbial rhodopsin (MRs) ion channels and pumps have become invaluable optogenetic tools for neuroscience as well as biomedical applications. Recently, MR-optogenetics expanded towards subcellular organelles opening principally new opportunities in optogenetic control of intracellular metabolism and signaling via precise manipulations of organelle ion gradients using light. This new optogenetic field expands the opportunities for basic and medical studies of cancer, cardiovascular, and metabolic disorders, providing more detailed and accurate control of cell physiology. This review summarizes recent advances in studies of the cellular metabolic processes and signaling mediated by optogenetic tools targeting mitochondria, endoplasmic reticulum (ER), lysosomes, and synaptic vesicles. Finally, we discuss perspectives of such an optogenetic approach in both fundamental and applied research.

Axonal organelle buildup from loss of AP-4 complex function causes exacerbation of amyloid plaque pathology and gliosis in Alzheimer's disease mouse model

Lysosomes and related precursor organelles robustly build up in swollen axons that surround amyloid plaques and disrupted axonal lysosome transport has been implicated in worsening Alzheimer's pathology. Our prior studies have revealed that loss of Adaptor protein-4 (AP-4) complex function, linked primarily to Spastic Paraplegia (HSP), leads to a similar build of lysosomes in structures we term "AP-4 dystrophies". Surprisingly, these AP-4 dystrophies were also characterized by enrichment of components of APP processing machinery, β-site cleaving enzyme 1 (BACE1) and Presenilin 2. Our studies examining whether the abnormal axonal lysosome build up resulting from AP-4 loss could lead to amyloidogenesis revealed that the loss of AP-4 complex function in an Alzheimer's disease model resulted in a strong increase in size and abundance of amyloid plaques in the hippocampus and corpus callosum as well as increased microglial association with the plaques. Interestingly, we found a further increase in enrichment of the secretase, BACE1, in the axonal swellings of the plaques of Alzheimer model mice lacking AP-4 complex compared to those having normal AP-4 complex function, suggestive of increased amyloidogenic processing under this condition. Additionally, the exacerbation of plaque pathology was region-specific as it did not increase in the cortex. The burden of the AP-4 linked axonal dystrophies/AP-4 dystrophies was higher in the corpus callosum and hippocampus compared to the cortex, establishing the critical role of AP-4 -dependent axonal lysosome transport and maturation in regulating amyloidogenic amyloid precursor protein processing.

Cellular Organelle-Related Transcriptomic Profile Abnormalities in Neuronopathic Types of Mucopolysaccharidosis: A Comparison with Other Neurodegenerative Diseases

Mucopolysaccharidoses (MPS) are a group of diseases caused by mutations in genes encoding lysosomal enzymes that catalyze reactions of glycosaminoglycan (GAG) degradation. As a result, GAGs accumulate in lysosomes, impairing the proper functioning of entire cells and tissues. There are 14 types/subtypes of MPS, which are differentiated by the kind(s) of accumulated GAG(s) and the type of a non-functional lysosomal enzyme. Some of these types (severe forms of MPS types I and II, MPS III, and MPS VII) are characterized by extensive central nervous system disorders. The aim of this work was to identify, using transcriptomic methods, organelle-related genes whose expression levels are changed in neuronopathic types of MPS compared to healthy cells while remaining unchanged in non-neuronopathic types of MPS. The study was conducted with fibroblast lines derived from patients with neuronopathic and non-neuronopathic types of MPS and control (healthy) fibroblasts. Transcriptomic analysis has identified genes related to cellular organelles whose expression is altered. Then, using fluorescence and electron microscopy, we assessed the morphology of selected structures. Our analyses indicated that the genes whose expression is affected in neuronopathic MPS are often associated with the structures or functions of the cell nucleus, endoplasmic reticulum, or Golgi apparatus. Electron microscopic studies confirmed disruptions in the structures of these organelles. Special attention was paid to up-regulated genes, such as PDIA3 and MFGE8, and down-regulated genes, such as ARL6IP6, ABHD5, PDE4DIP, YIPF5, and CLDN11. Of particular interest is also the GM130 (GOLGA2) gene, which encodes golgin A2, which revealed an increased expression in neuronopathic MPS types. We propose to consider the levels of mRNAs of these genes as candidates for biomarkers of neurodegeneration in MPS. These genes may also become potential targets for therapies under development for neurological disorders associated with MPS and candidates for markers of the effectiveness of these therapies. Although fibroblasts rather than nerve cells were used in this study, it is worth noting that potential genetic markers characteristic solely of neurons would be impractical in testing patients, contrary to somatic cells that can be relatively easily obtained from assessed persons.

Intersectin - many facets of a scaffold protein

Intersectin (ITSN) is a multi-domain scaffold protein with a diverse array of functions including regulation of endocytosis, vesicle transport, and activation of various signal transduction pathways. There are two ITSN genes located on chromosomes 21 and 2 encoding for proteins ITSN1 and ITSN2, respectively. Each ITSN gene encodes two major isoforms, ITSN-Long (ITSN-L) and ITSN-Short (ITSN-S), due to alternative splicing. ITSN1 and 2, collectively referred to as ITSN, are implicated in many physiological and pathological processes, such as neuronal maintenance, actin cytoskeletal rearrangement, and tumor progression. ITSN is mis-regulated in many tumors, such as breast, lung, neuroblastomas, and gliomas. Altered expression of ITSN is also found in several neurodegenerative diseases, such as Down Syndrome and Alzheimer's disease. This review summarizes recent studies on ITSN and provides an overview of the function of this important family of scaffold proteins in various biological processes.

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.

ApoE4 expression disrupts tau uptake, trafficking, and clearance in astrocytes

Tauopathies are a collection of neurodegenerative diseases characterized by the accumulation of pathogenic aggregates of the microtubule-associated protein tau. Despite the prevalence and diversity of tau astrogliopathy in tauopathies, the interactions between astrocytes and tau in the brain, and the influence of neurodegenerative genetic risk factors like the apolipoprotein E4 (apoE4) isoform, are largely unknown. Here, we leveraged primary and immortalized astrocytes expressing humanized apoE isoforms to characterize the mechanisms by which astrocytes interact with and eliminate extracellular tau, and the influence of apoE genotype on these processes. Our work indicates that astrocytes rapidly internalize, process, and release tau via an exosomal secretory mechanism under physiological conditions. However, we found that apoE4 disrupted these processes in comparison to apoE3, resulting in an astrocytic phenotype prone to intracellular tau accumulation. Furthermore, exposure to repetitive mild traumatic brain injuries exacerbated the apoE4-induced impairments in tau processing and elimination by astrocytes in apoE4 targeted-replacement mice. The diminished ability of apoE4 astrocytes to eliminate extracellular tau can lead to an accumulation of pathogenic tau, which induces mitochondrial dysfunction, as demonstrated by our studies. In total, our findings suggest that the apoE4 isoform lowers the threshold of astrocytic resilience to pathogenic tau, rendering them susceptible to bioenergetic deficits in the early stages of neurodegenerative diseases such as traumatic brain injury, potentially contributing to neurological decline.

Organelle mapping in dendrites of human iPSC-derived neurons reveals dynamic functional dendritic Golgi structures

Secretory pathways within dendrites of neurons have been proposed for local transport of newly synthesized proteins. However, little is known about the dynamics of the local secretory system and whether the organelles are transient or stable structures. Here, we quantify the spatial and dynamic behavior of dendritic Golgi and endosomes during differentiation of human neurons generated from induced pluripotent stem cells (iPSCs). In early neuronal development, before and during migration, the entire Golgi apparatus transiently translocates from the soma into dendrites. In mature neurons, dynamic Golgi elements, containing cis and trans cisternae, are transported from the soma along dendrites, in an actin-dependent process. Dendritic Golgi outposts are dynamic and display bidirectional movement. Similar structures were observed in cerebral organoids. Using the retention using selective hooks (RUSH) system, Golgi resident proteins are transported efficiently into Golgi outposts from the endoplasmic reticulum. This study reveals dynamic, functional Golgi structures in dendrites and a spatial map for investigating dendrite trafficking in human neurons.

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.

Commentary: BAG3 as a Mediator of Endosome Function and Tau Clearance

Tauopathies are a group of heterogeneous neurodegenerative conditions characterized by the deposition of abnormal tau protein in the brain. The underlying mechanisms that contribute to the accumulation of tau in these neurodegenerative diseases are multifactorial; nonetheless, there is a growing awareness that dysfunction of endosome-lysosome pathways is a pivotal factor. BCL2 associated athanogene 3 (BAG3) is a multidomain protein that plays a key role in maintaining neuronal proteostasis. Further, recent data indicate that BAG3 plays an important role in mediating vacuolar-dependent degradation of tau. Overexpression of BAG3 in a tauopathy mouse model decreased pathological tau levels and alleviated synapse loss. High throughput screens of BAG3 interactors have identified key players in the vacuolar system; these include clathrin and regulators of small GTPases. These findings suggest that BAG3 is an important regulator of endocytic pathways. In this commentary, we discuss the potential mechanisms by which BAG3 regulates the vacuolar system and tau proteostasis.

Constant Conversion Rate of Endolysosomes Revealed by a pH-Sensitive Fluorescent Probe

Endolysosome dynamics plays an important role in autophagosome biogenesis. Hence, imaging the subcellular dynamics of endolysosomes using high-resolution fluorescent imaging techniques would deepen our understanding of autophagy and benefit the development of pharmaceuticals against endosome-related diseases. Taking advantage of the intramolecular charge-transfer mechanism, herein we report a cationic quinolinium-based fluorescent probe (PyQPMe) that exhibits excellent pH-sensitive fluorescence in endolysosomes at different stages of interest. A systematic photophysical and computational study on PyQPMe was carried out to rationalize its highly pH-dependent absorption and emission spectra. The large Stokes shift and strong fluorescence intensity of PyQPMe can effectively reduce the background noise caused by excitation light and microenvironments and provide a high signal-to-noise ratio for high-resolution imaging of endolysosomes. By applying PyQPMe as a small molecular probe in live cells, we were able to reveal a constant conversion rate from early endosomes to late endosomes/lysosomes during autophagy at the submicron level.

3, 14, 19-Triacetyl Andrographolide alleviates the cognitive dysfunction of 3 × Tg-AD mice by inducing initiation and promoting degradation process of autophagy

The present study aims to investigate the cognition-enhancing effect of 3, 14, 19-Triacetyl andrographolide (ADA) on learning and memory deficits in 3 × Tg-AD mice and to explore its underlying mechanism. Eight-month-old 3 × Tg-AD mice and C57BL/6J mice were randomly divided into three groups, namely wild-type group, 3 × Tg-AD group, and 3 × Tg-AD+ADA group (5 mg/kg, for 21 days, i.p.). We found that ADA significantly improved learning and cognition impairment, inhibited the loss of Nissl body, and reduced Aβ load in the brains of 3 × Tg-AD mice. In addition, ADA enhanced the levels of PSD95 and SYP, which were closely associated with synaptic plasticity. Accumulated autophagosomes, LC3II, and P62 in hippocampus and cortex of 3 × Tg-AD mice were decreased by ADA treatment. Furthermore, ADA administration further down-regulated the expressions of p-AKT and p-mTOR, reduced the level of CTSB, and increased the co-localization of LC3 and LAMP1 in the brains of 3 × Tg-AD mice, implying that ADA-induced autophagy initiation and also promoted the degradation process. In Aβ_25-35 -induced HT22 cells, ADA displayed similar effects on autophagy flux as observed in 3 × Tg-AD mice. Our finding verified that ADA could improve synaptic plasticity and cognitive function, which is mainly attributed to the key roles of ADA in autophagy induction and degradation.

The Pursuit of the "Inside" of the Amyloid Hypothesis-Is C99 a Promising Therapeutic Target for Alzheimer's Disease?

Aducanumab, co-developed by Eisai (Japan) and Biogen (U.S.), has received Food and Drug Administration approval for treating Alzheimer's disease (AD). In addition, its successor antibody, lecanemab, has been approved. These antibodies target the aggregated form of the small peptide, amyloid-β (Aβ), which accumulates in the patient brain. The "amyloid hypothesis" based therapy that places the aggregation and toxicity of Aβ at the center of the etiology is about to be realized. However, the effects of immunotherapy are still limited, suggesting the need to reconsider this hypothesis. Aβ is produced from a type-I transmembrane protein, Aβ precursor protein (APP). One of the APP metabolites, the 99-amino acids C-terminal fragment (C99, also called βCTF), is a direct precursor of Aβ and accumulates in the AD patient's brain to demonstrate toxicity independent of Aβ. Conventional drug discovery strategies have focused on Aβ toxicity on the "outside" of the neuron, but C99 accumulation might explain the toxicity on the "inside" of the neuron, which was overlooked in the hypothesis. Furthermore, the common region of C99 and Aβ is a promising target for multifunctional AD drugs. This review aimed to outline the nature, metabolism, and impact of C99 on AD pathogenesis and discuss whether it could be a therapeutic target complementing the amyloid hypothesis.

Beneficial Effect of a Small Pharmacologic Chaperone on the Established Alzheimer's Disease Phenotype

BACKGROUND

The endosomal retromer complex system is a key controller for trafficking of proteins. Downregulation of its recognition core proteins, such as VPS35, is present in Alzheimer's disease (AD) brain, whereas its normalization prevents the development of AD pathology in a transgenic model with amyloid-β deposits and tau tangles.

OBJECTIVE

Assess the effect of targeting VPS35 after the AD pathology and memory impairments have developed.

METHODS

Twelve-month-old triple transgenic mice were treated with a small pharmacological chaperone, TPT-172, or vehicle for 14 weeks. At the end of this period, the effect of the drug on their phenotype was evaluated.

RESULTS

While control mice had a decline of learning and memory, the group receiving the chaperone did not. Moreover, when compared with controls the treated mice had significantly less amyloid-β peptides and phosphorylated tau, elevation of post-synaptic protein, and reduction in astrocytes activation.

CONCLUSION

Taken together, our findings demonstrate that pharmacologic stabilization of the retromer recognition core is beneficial also after the AD-like pathologic phenotype is established.

Loss of small GTPase Rab7 activation in prion infection negatively affects a feedback loop regulating neuronal cholesterol metabolism

Prion diseases are fatal and infectious neurodegenerative diseases that occur in humans and animals. They are caused by the misfolding of the cellular prion protein PrP^c into the infectious isoform PrP^Sc. PrP^Sc accumulates mostly in endolysosomal vesicles of prion-infected cells, eventually causing neurodegeneration. In response to prion infection, elevated cholesterol levels and a reduction in membrane-attached small GTPase Rab7 have been observed in neuronal cells. Here, we investigated the molecular events causing an impaired Rab7 membrane attachment and the potential mechanistic link with elevated cholesterol levels in prion infection. We demonstrate that prion infection is associated with reduced levels of active Rab7 (Rab7.GTP) in persistently prion-infected neuronal cell lines, primary cerebellar granular neurons, and neurons in the brain of mice with terminal prion disease. In primary cerebellar granular neurons, levels of active Rab7 were increased during the very early stages of the prion infection prior to a significant decrease concomitant with PrP^Sc accumulation. The reduced activation of Rab7 in prion-infected neuronal cell lines is also associated with its reduced ubiquitination status, decreased interaction with its effector RILP, and altered lysosomal positioning. Consequently, the Rab7-mediated trafficking of low-density lipoprotein to lysosomes is delayed. This results in an impaired feedback regulation of cholesterol synthesis leading to an increase in cholesterol levels. Notably, transient overexpression of the constitutively active mutant of Rab7 rescues the delay in the low-density lipoprotein trafficking, hence reducing cholesterol levels and attenuating PrP^Sc propagation, demonstrating a mechanistic link between the loss of Rab7.GTP and elevated cholesterol levels.

CRISPRi screening reveals regulators of tau pathology shared between exosomal and vesicle-free tau

The aggregation of the microtubule-associated protein tau is a defining feature of Alzheimer's disease and other tauopathies. Tau pathology is believed to be driven by free tau aggregates and tau carried within exosome-like extracellular vesicles, both of which propagate trans-synaptically and induce tau pathology in recipient neurons by a corrupting process of seeding. Here, we performed a genome-wide CRISPRi screen in tau biosensor cells and identified cellular regulators shared by both mechanisms of tau seeding. We identified ANKLE2, BANF1, NUSAP1, EIF1AD, and VPS18 as the top validated regulators that restrict tau aggregation initiated by both exosomal and vesicle-free tau seeds. None of our validated hits affected the uptake of either form of tau seeds, supporting the notion that they operate through a cell-autonomous mechanism downstream of the seed uptake. Lastly, validation studies with human brain tissue also revealed that several of the identified protein hits are down-regulated in the brains of Alzheimer's patients, suggesting that their decreased activity may be required for the emergence or progression of tau pathology in the human brain.

Early lysosome defects precede neurodegeneration with amyloid-β and tau aggregation in NHE6-null rat brain

Loss-of-function mutations in the X-linked endosomal Na+/H+ exchanger 6 (NHE6) cause Christianson syndrome in males. Christianson syndrome involves endosome dysfunction leading to early cerebellar degeneration, as well as later-onset cortical and subcortical neurodegeneration, potentially including tau deposition as reported in post-mortem studies. In addition, there is reported evidence of modulation of amyloid-β levels in experimental models wherein NHE6 expression was targeted. We have recently shown that loss of NHE6 causes defects in endosome maturation and trafficking underlying lysosome deficiency in primary mouse neurons in vitro. For in vivo studies, rat models may have an advantage over mouse models for the study of neurodegeneration, as rat brain can demonstrate robust deposition of endogenously-expressed amyloid-β and tau in certain pathological states. Mouse models generally do not show the accumulation of insoluble, endogenously-expressed (non-transgenic) tau or amyloid-β. Therefore, to study neurodegeneration in Christianson syndrome and the possibility of amyloid-β and tau pathology, we generated an NHE6-null rat model of Christianson syndrome using CRISPR-Cas9 genome-editing. Here, we present the sequence of pathogenic events in neurodegenerating NHE6-null male rat brains across the lifespan. NHE6-null rats demonstrated an early and rapid loss of Purkinje cells in the cerebellum, as well as a more protracted neurodegenerative course in the cerebrum. In both the cerebellum and cerebrum, lysosome deficiency is an early pathogenic event, preceding autophagic dysfunction. Microglial and astrocyte activation also occur early. In the hippocampus and cortex, lysosome defects precede loss of pyramidal cells. Importantly, we subsequently observed biochemical and in situ evidence of both amyloid-β and tau aggregation in the aged NHE6-null hippocampus and cortex (but not in the cerebellum). Tau deposition is widely distributed, including cortical and subcortical distributions. Interestingly, we observed tau deposition in both neurons and glia, as has been reported in Christianson syndrome post-mortem studies previously. In summary, this experimental model is among very few examples of a genetically modified animal that exhibits neurodegeneration with deposition of endogenously-expressed amyloid-β and tau. This NHE6-null rat will serve as a new robust model for Christianson syndrome. Furthermore, these studies provide evidence for linkages between endolysosome dysfunction and neurodegeneration involving protein aggregations, including amyloid-β and tau. Therefore these studies may provide insight into mechanisms of more common neurodegenerative disorders, including Alzheimer's disease and related dementias.

A genetically modified minipig model for Alzheimer’s disease with SORL1 haploinsufficiency

The established causal genes in Alzheimer’s disease (AD), APP, PSEN1, and PSEN2, are functionally characterized using biomarkers, capturing an in vivo profile reflecting the disease’s initial preclinical phase. Mutations in SORL1, encoding the endosome recycling receptor SORLA, are found in 2%–3% of individuals with early-onset AD, and SORL1 haploinsufficiency appears to be causal for AD. To test whether SORL1 can function as an AD causal gene, we use CRISPR-Cas9-based gene editing to develop a model of SORL1 haploinsufficiency in Göttingen minipigs, taking advantage of porcine models for biomarker investigations. SORL1 haploinsufficiency in young adult minipigs is found to phenocopy the preclinical in vivo profile of AD observed with APP, PSEN1, and PSEN2, resulting in elevated levels of β-amyloid (Aβ) and tau preceding amyloid plaque formation and neurodegeneration, as observed in humans. Our study provides functional support for the theory that SORL1 haploinsufficiency leads to endosome cytopathology with biofluid hallmarks of autosomal dominant AD.

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Minipig model of Alzheimer’s disease by CRISPR knockout of the causal gene SORL1

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Young SORL1 het minipigs phenocopy a preclinical CSF biomarker profile of individuals with AD

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SORL1 haploinsufficiency causes enlarged endosomes similar to neuronal AD pathology

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A minipig model bridging the translational gap between AD mouse models and affected individuals

Molecular Mechanism and Regulation of Autophagy and Its Potential Role in Epilepsy

Autophagy is an evolutionally conserved degradation mechanism for maintaining cell homeostasis whereby cytoplasmic components are wrapped in autophagosomes and subsequently delivered to lysosomes for degradation. This process requires the concerted actions of multiple autophagy-related proteins and accessory regulators. In neurons, autophagy is dynamically regulated in different compartments including soma, axons, and dendrites. It determines the turnover of selected materials in a spatiotemporal control manner, which facilitates the formation of specialized neuronal functions. It is not surprising, therefore, that dysfunctional autophagy occurs in epilepsy, mainly caused by an imbalance between excitation and inhibition in the brain. In recent years, much attention has been focused on how autophagy may cause the development of epilepsy. In this article, we overview the historical landmarks and distinct types of autophagy, recent progress in the core machinery and regulation of autophagy, and biological roles of autophagy in homeostatic maintenance of neuronal structures and functions, with a particular focus on synaptic plasticity. We also discuss the relevance of autophagy mechanisms to the pathophysiology of epileptogenesis.

Aβ42 oligomers trigger synaptic loss through CAMKK2-AMPK-dependent effectors coordinating mitochondrial fission and mitophagy

During the early stages of Alzheimer's disease (AD) in both mouse models and human patients, soluble forms of Amyloid-β 1-42 oligomers (Aβ42o) trigger loss of excitatory synapses (synaptotoxicity) in cortical and hippocampal pyramidal neurons (PNs) prior to the formation of insoluble amyloid plaques. In a transgenic AD mouse model, we observed a spatially restricted structural remodeling of mitochondria in the apical tufts of CA1 PNs dendrites corresponding to the dendritic domain where the earliest synaptic loss is detected in vivo. We also observed AMPK over-activation as well as increased fragmentation and loss of mitochondrial biomass in Ngn2-induced neurons derived from a new APP^Swe/Swe knockin human ES cell line. We demonstrate that Aβ42o-dependent over-activation of the CAMKK2-AMPK kinase dyad mediates synaptic loss through coordinated phosphorylation of MFF-dependent mitochondrial fission and ULK2-dependent mitophagy. Our results uncover a unifying stress-response pathway causally linking Aβ42o-dependent structural remodeling of dendritic mitochondria to synaptic loss.

Copolymer-1 as a potential therapy for mild cognitive impairment

Mild cognitive impairment (MCI) is a prodromal stage of memory impairment that may precede dementia. MCI is classified by the presence or absence of memory impairment into amnestic or non-amnestic MCI, respectively. More than 90% of patients with amnestic MCI who progress towards dementia meet criteria for Alzheimer's disease (AD). A combination of mechanisms promotes MCI, including intracellular neurofibrillary tangle formation, extracellular amyloid deposition, oxidative stress, neuronal loss, synaptodegeneration, cholinergic dysfunction, cerebrovascular disease, and neuroinflammation. However, emerging evidence indicates that neuroinflammation plays an important role in the pathogenesis of cognitive impairment. Unfortunately, there are currently no Food and Drug Administration (FDA)-approved drugs for MCI. Copolymer-1 (Cop-1), also known as glatiramer acetate, is a synthetic polypeptide of four amino acids approved by the FDA for the treatment of relapsing-remitting multiple sclerosis. Cop-1 therapeutic effect is attributed to immunomodulation, promoting a switch from proinflammatory to anti-inflammatory phenotype. In addition to its anti-inflammatory properties, it stimulates brain-derived neurotrophic factor (BDNF) secretion, a neurotrophin involved in neurogenesis and the generation of hippocampal long-term potentials. Moreover, BDNF levels are significantly decreased in patients with cognitive impairment. Therefore, Cop-1 immunization might promote synaptic plasticity and memory consolidation by increasing BDNF production in patients with MCI.

Defective mitophagy and the etiopathogenesis of Alzheimer's disease

Accumulation of impaired mitochondria and energy metabolism disorders are non-negligible features of both aging and age-related neurodegeneration, including Alzheimer’s disease (AD). A growing number of studies suggest that mitophagy disorders play an important role in AD occurrence and development. The interaction between mitophagy deficits and Aβ or Tau pathology may form a vicious cycle and cause neuronal damage and death. Elucidating the molecular mechanism of mitophagy and its role in AD may provide insights into the etiology and mechanisms of AD. Defective mitophagy is a potential target for AD prevention and treatment.

Glucose-lowering drugs, cognition, and dementia: The clinical evidence

Type 2 diabetes mellitus (T2DM) is an important risk factor for dementia. The possibility to mitigate this risk by controlling T2DM is compelling; however, different glucose-lowering drugs have different effects on the brain by virtue of their different mechanisms of action. The clinical and epidemiological data appear mixed, warranting careful critical evaluation of the human studies. Here we examine the evidence in the context of dementia prevention and treatment, both for people with and without T2DM. We discuss the evidence on this scaffold of research directions, identifying methodological complexities in the extant literature (e.g. comparator discrepancies, changes in the therapeutic landscape), and the implications of different outcome measures (e.g. neuropsychological). We consider possible implications of cerebrovascular protection vs. effects on progression of neurodegenerative proteinopathy, and we present a research roadmap for glucose-lowering drugs in cognitive neurology, including neuroimaging, and fluid biomarkers. We conclude that there is great potential to advance personalized strategies to prevent and treat dementia with glucose-lowering drugs.

Roles of Endomembrane Alkali Cation/Proton Exchangers in Synaptic Function and Neurodevelopmental Disorders

Endomembrane alkali cation (Na⁺, K⁺)/proton (H⁺) exchangers (eNHEs) are increasingly associated with neurological disorders. These eNHEs play integral roles in regulating the luminal pH, processing, and trafficking of cargo along the secretory (Golgi and post-Golgi vesicles) and endocytic (early, recycling, and late endosomes) pathways, essential regulatory processes vital for neuronal development and plasticity. Given the complex morphology and compartmentalization of multipolar neurons, the contribution of eNHEs in maintaining optimal pH homeostasis and cargo trafficking is especially significant during periods of structural and functional development and remodeling. While the importance of eNHEs has been demonstrated in a variety of non-neuronal cell types, their involvement in neuronal function is less well understood. In this review, we will discuss their emerging roles in excitatory synaptic function, particularly as it pertains to cellular learning and remodeling. We will also explore their connections to neurodevelopmental conditions, including intellectual disability, autism, and attention deficit hyperactivity disorders.

Ras and Rab Interactor 3: From Cellular Mechanisms to Human Diseases

Ras and Rab interactor 3 (RIN3) functions as a Guanine nucleotide Exchange Factor (GEF) for some members of the Rab family of small GTPase. By promoting the activation of Rab5, RIN3 plays an important role in regulating endocytosis and endocytic trafficking. In addition, RIN3 activates Ras, another small GTPase, that controls multiple signaling pathways to regulate cellular function. Increasing evidence suggests that dysregulation of RIN3 activity may contribute to the pathogenesis of several disease conditions ranging from Paget’s Disease of the Bone (PDB), Alzheimer’s Disease (AD), Chronic Obstructive Pulmonary Disease (COPD) and to obesity. Recent genome-wide association studies (GWAS) identified variants in the RIN3 gene to be linked with these disease conditions. Interestingly, some variants appear to be missense mutations in the functional domains of the RIN3 protein while most variants are located in the noncoding regions of the RIN3 gene, potentially altering its gene expression. However, neither the protein structure of RIN3 nor its exact function(s) (except for its GEF activity) has been fully defined. Furthermore, how the polymorphisms/variants contribute to disease pathogenesis remain to be understood. Herein, we examine, and review published studies in an attempt to provide a better understanding of the physiological function of RIN3; More importantly, we construct a framework linking the polymorphisms/variants of RIN3 to altered cell signaling and endocytic traffic, and to potential disease mechanism(s).

ApoE Cascade Hypothesis in the pathogenesis of Alzheimer's disease and related dementias

The ε4 allele of the apolipoprotein E gene (APOE4) is a strong genetic risk factor for Alzheimer's disease (AD) and several other neurodegenerative conditions, including Lewy body dementia (LBD). The three APOE alleles encode protein isoforms that differ from one another only at amino acid positions 112 and 158: apoE2 (C112, C158), apoE3 (C112, R158), and apoE4 (R112, R158). Despite progress, it remains unclear how these small amino acid differences in apoE sequence among the three isoforms lead to profound effects on aging and disease-related pathways. Here, we propose a novel "ApoE Cascade Hypothesis" in AD and age-related cognitive decline, which states that the biochemical and biophysical properties of apoE impact a cascade of events at the cellular and systems levels, ultimately impacting aging-related pathogenic conditions including AD. As such, apoE-targeted therapeutic interventions are predicted to be more effective by addressing the biochemical phase of the cascade.

The neuronal-specific isoform of BIN1 regulates β-secretase cleavage of APP and Aβ generation in a RIN3-dependent manner

Genome-wide association studies have identified BIN1 (Bridging integrator 1) and RIN3 (Ras and Rab interactor 3) as genetic risk factors for late-onset Alzheimer's disease (LOAD). The neuronal isoform of BIN1 (BIN1V1), but not the non-neuronal isoform (BIN1V9), has been shown to regulate tau-pathology and Aβ generation via RAB5-mediated endocytosis in neurons. BIN1 directly interacts with RIN3 to initiate RAB5-mediated endocytosis, which is essential for β-secretase (BACE1)-mediated β-secretase cleavage of β-amyloid precursor protein (APP) to generate Amyloid-β (Aβ), the key component of senile plaques in AD. Understanding the regulatory roles of BIN1 (neuronal BIN1V1) and RIN3 in β-secretase mediated cleavage of APP and Aβ generation is key to developing novel therapeutics to delay or prevent AD progression. Neuronal and non-neuronal isoforms of BIN1 (BIN1V1 and BIN1V9, respectively) were introduced with RIN3 into an in vitro cell-based system to test RIN3-dependent effects of neuronal BIN1V1 and non-neuronal BIN1V9 on β-secretase-mediated cleavage of APP and Aβ generation. Confocal microscopy was performed to examine RIN3-dependent subcellular localization of BIN1V1 and BIN1V9. Western blot analysis was performed to assess the effects of RIN3 and BIN1V1/BIN1V9 on β-secretase mediated processing of APP. We enriched cells expressing BIN1V1 without or with RIN3 via FACS to measure Aβ generation using Aβ ELISA assay, and to evaluate APP internalization by chasing biotinylated or antibody-labeled cell surface APP. Neuronal BIN1V1 containing the CLAP domain and non-neuronal BIN1V9 lacking the CLAP domain are the major isoforms present in the brain. Employing confocal microscopy, we showed that RIN3 differentially regulates the recruitment of both BIN1V1 and BIN1V9 into RAB5-endosomes. We further showed that BIN1V1, but not BIN1V9, downregulates β-secretase (BACE1)-mediated processing of APP in a RIN3-dependent manner. Overexpression of BIN1V1 also attenuated Aβ generation in a RIN3-dependent manner. Using cell-based internalization assays, we show BIN1V1, but not BIN1V9, delays the endocytosis of APP, but not of BACE1, into early endosomes, thereby spatially and temporally separating these two proteins into different cellular compartments, resulting in reduced cleavage of APP by BACE1 and reduced Aβ generation-all in a RIN3-dependent manner. Finally, we show that RIN3 sequesters BIN1V1 in RAB5-positive early endosomes, likely via the CLAP-domain, resulting in attenuated β-secretase processing of APP and Aβ generation by delaying endocytosis of APP. Our findings provide new mechanistic data on how two AD-associated molecules, RIN3 and BIN1 (neuronal BIN1V1), interact to govern Aβ production, implicating these two proteins as potential therapeutic targets for the prevention and treatment of AD.

The neuronal retromer can regulate both neuronal and microglial phenotypes of Alzheimer's disease

Disruption of retromer-dependent endosomal trafficking is considered pathogenic in late-onset Alzheimer's disease (AD). Here, to investigate this disruption in the intact brain, we turn to a genetic mouse model where the retromer core protein VPS35 is depleted in hippocampal neurons, and then we replete VPS35 using an optimized viral vector protocol. The VPS35 depletion-repletion studies strengthen the causal link between the neuronal retromer and AD-associated neuronal phenotypes, including the acceleration of amyloid precursor protein cleavage and the loss of synaptic glutamate receptors. Moreover, the studies show that the neuronal retromer can regulate a distinct, dystrophic, microglia morphology, phenotypic of hippocampal microglia in AD. Finally, the neuronal and, in part, the microglia responses to VPS35 depletion were found to occur independent of tau. Showing that the neuronal retromer can regulate AD-associated pathologies in two of AD's principal cell types strengthens the link, and clarifies the mechanism, between endosomal trafficking and late-onset sporadic AD.

JIP3 links lysosome transport to regulation of multiple components of the axonal cytoskeleton

Lysosome axonal transport is important for the clearance of cargoes sequestered by the endocytic and autophagic pathways. Building on observations that mutations in the JIP3 (MAPK8IP3) gene result in lysosome-filled axonal swellings, we analyzed the impact of JIP3 depletion on the cytoskeleton of human neurons. Dynamic focal lysosome accumulations were accompanied by disruption of the axonal periodic scaffold (spectrin, F-actin and myosin II) throughout each affected axon. Additionally, axonal microtubule organization was locally disrupted at each lysosome-filled swelling. This local axonal microtubule disorganization was accompanied by accumulations of both F-actin and myosin II. These results indicate that transport of axonal lysosomes is functionally interconnected with mechanisms that control the organization and maintenance of the axonal cytoskeleton. They have potential relevance to human neurological disease arising from JIP3 mutations as well as for neurodegenerative diseases associated with the focal accumulations of lysosomes within axonal swellings such as Alzheimer’s disease.

Rafiq et al. report that disruption of JIP3-dependent control of axonal lysosome transport in human neurons results in unexpected changes to the organization of multiple cytoskeletal proteins. This study provides new insights that improve our understanding of intellectual disabilities caused by mutations in JIP3, and are relevant for neurodegenerative diseases associated with accumulations of lysosomes such as the Alzheimer’s disease

Rab35 and glucocorticoids regulate APP and BACE1 trafficking to modulate Aβ production

Chronic stress and elevated glucocorticoids (GCs), the major stress hormones, are risk factors for Alzheimer’s disease (AD) and promote AD pathomechanisms, including overproduction of toxic amyloid-β (Aβ) peptides and intraneuronal accumulation of hyperphosphorylated Tau protein. The latter is linked to downregulation of the small GTPase Rab35, which mediates Tau degradation via the endolysosomal pathway. Whether Rab35 is also involved in Aβ overproduction remains an open question. Here, we find that hippocampal Rab35 levels are decreased not only by stress/GC but also by aging, another AD risk factor. Moreover, we show that Rab35 negatively regulates Aβ production by sorting amyloid precursor protein (APP) and β-secretase (BACE1) out of the endosomal network, where they interact to produce Aβ. Interestingly, Rab35 coordinates distinct intracellular trafficking steps for BACE1 and APP, mediated by its effectors OCRL and ACAP2, respectively. Finally, we demonstrate that Rab35 overexpression prevents the amyloidogenic trafficking of APP and BACE1 induced by high GC levels. These studies identify Rab35 as a key regulator of APP processing and suggest that its downregulation may contribute to stress-related and AD-related amyloidogenesis.

MicroRNA-Target Interaction Regulatory Network in Alzheimer's Disease

Alzheimer’s Disease (AD) is a progressive neurodegenerative disorder and the most common cause of dementia; however, early diagnosis of the disease is challenging. Research suggests that biomarkers found in blood, such as microRNAs (miRNA), may be promising for AD diagnostics. Experimental data on miRNA–target interactions (MTI) associated with AD are scattered across databases and publications, thus making the identification of promising miRNA biomarkers for AD difficult. In response to this, a list of experimentally validated AD-associated MTIs was obtained from miRTarBase. Cytoscape was used to create a visual MTI network. STRING software was used for protein–protein interaction analysis and mirPath was used for pathway enrichment analysis. Several targets regulated by multiple miRNAs were identified, including: BACE1, APP, NCSTN, SP1, SIRT1, and PTEN. The miRNA with the highest numbers of interactions in the network were: miR-9, miR-16, miR-34a, miR-106a, miR-107, miR-125b, miR-146, and miR-181c. The analysis revealed seven subnetworks, representing disease modules which have a potential for further biomarker development. The obtained MTI network is not yet complete, and additional studies are needed for the comprehensive understanding of the AD-associated miRNA targetome.

Macroautophagy and aging: The impact of cellular recycling on health and longevity

Aging is associated with many deleterious changes at the cellular level, including the accumulation of potentially toxic components that can have devastating effects on health. A key protective mechanism to this end is the cellular recycling process called autophagy. During autophagy, damaged or surplus cellular components are delivered to acidic vesicles called lysosomes, that secure degradation and recycling of the components. Numerous links between autophagy and aging exist. Autophagy declines with age, and increasing evidence suggests that this reduction plays important roles in both physiological aging and the development of age-associated disorders. Studies in pharmacologically and genetically manipulated model organisms indicate that defects in autophagy promote age-related diseases, and conversely, that enhancement of autophagy has beneficial effects on both healthspan and lifespan. Here, we review our current understanding of the role of autophagy in different physiological processes and their molecular links with aging and age-related diseases. We also highlight some recent advances in the field that could accelerate the development of autophagy-based therapeutic interventions.

Alzheimer's vulnerable brain region relies on a distinct retromer core dedicated to endosomal recycling

Whether and how the pathogenic disruptions in endosomal trafficking observed in Alzheimer’s disease (AD) are linked to its anatomical vulnerability remain unknown. Here, we began addressing these questions by showing that neurons are enriched with a second retromer core, organized around VPS26b, differentially dedicated to endosomal recycling. Next, by imaging mouse models, we show that the trans-entorhinal cortex, a region most vulnerable to AD, is most susceptible to VPS26b depletion—a finding validated by electrophysiology, immunocytochemistry, and behavior. VPS26b was then found enriched in the trans-entorhinal cortex of human brains, where both VPS26b and the retromer-related receptor SORL1 were found deficient in AD. Finally, by regulating glutamate receptor and SORL1 recycling, we show that VPS26b can mediate regionally selective synaptic dysfunction and SORL1 deficiency. Together with the trans-entorhinal’s unique network properties, hypothesized to impose a heavy demand on endosomal recycling, these results suggest a general mechanism that can explain AD’s regional vulnerability.

Trans-entorhinal cortex neurons are most vulnerable to Alzheimer’s disease. Simoes et al. explain this vulnerability by showing that these neurons are dependent on a distinct VPS26b-retromer core differentially dedicated to endosomal recycling. VPS26b is highly expressed in these neurons, where they regulate synaptic function, GluA1/SORL1 recycling, and disease-associated pathologies

Carbon Nanotube Exposure Triggers a Cerebral Peptidomic Response: Barrier Compromise, Neuroinflammation, and a Hyperexcited State

The unique physicochemical properties of carbon nanomaterials and their ever-growing utilization generate a serious concern for occupational risk. Pulmonary exposure to these nanoparticles induces local and systemic inflammation, cardiovascular dysfunction, and even cognitive deficits. Although multiple routes of extrapulmonary toxicity have been proposed, the mechanism for and manner of neurologic effects remain minimally understood. Here, we examine the cerebral spinal fluid (CSF)-derived peptidomic fraction as a reflection of neuropathological alterations induced by pulmonary carbon nanomaterial exposure. Male C57BL/6 mice were exposed to 10 or 40 µg of multiwalled carbon nanotubes (MWCNT) by oropharyngeal aspiration. Serum and CSFs were collected 4 h post exposure. An enriched peptide fraction of both biofluids was analyzed using ion mobility-enabled data-independent mass spectrometry for label-free quantification. MWCNT exposure induced a prominent peptidomic response in the blood and CSF; however, correlation between fluids was limited. Instead, we determined that a MWCNT-induced peptidomic shift occurred specific to the CSF with 292 significant responses found that were not in serum. Identified MWCNT-responsive peptides depicted a mechanism involving aberrant fibrinolysis (fibrinopeptide A), blood-brain barrier permeation (homeobox protein A4), neuroinflammation (transmembrane protein 131L) with reactivity by astrocytes and microglia, and a pro-degradative (signal transducing adapter molecule, phosphoglycerate kinase), antiplastic (AF4/FMR2 family member 1, vacuolar protein sorting-associated protein 18) state with the excitation-inhibition balance shifted to a hyperexcited (microtubule-associated protein 1B) phenotype. Overall, the significant pathologic changes observed were consistent with early neurodegenerative disease and were diagnostically reflected in the CSF peptidome.

Endosomal-lysosomal dysfunctions in Alzheimer's disease: Pathogenesis and therapeutic interventions

The endosomal-lysosomal system mediates the process of protein degradation through endocytic pathway. This system consists of early endosomes, late endosomes, recycling endosomes and lysosomes. Each component in the endosomal-lysosomal system plays individual crucial role and they work concordantly to ensure protein degradation can be carried out functionally. Dysregulation in the endosomal-lysosomal system can contribute to the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD). In AD endosomal-lysosomal abnormalities are the earliest pathological features to note and hence it is important to understand the involvement of endosomal-lysosomal dysfunction in the pathogenesis of AD. In-depth understanding of this dysfunction can allow development of new therapeutic intervention to prevent and treat AD.

Multispectral Fingerprinting Resolves Dynamics of Nanomaterial Trafficking in Primary Endothelial Cells

Intracellular vesicle trafficking involves a complex series of biological pathways used to sort, recycle, and degrade extracellular components, including engineered nanomaterials (ENMs) which gain cellular entry via active endocytic processes. A recent emphasis on routes of ENM uptake has established key physicochemical properties which direct certain mechanisms, yet relatively few studies have identified their effect on intracellular trafficking processes past entry and initial subcellular localization. Here, we developed and applied an approach where single-walled carbon nanotubes (SWCNTs) play a dual role-that of an ENM undergoing intracellular processing, in addition to functioning as the signal transduction element reporting these events in individual cells with single organelle resolution. We used the exceptional optical properties exhibited by noncovalent hybrids of single-stranded DNA and SWCNTs (DNA-SWCNTs) to report the progression of intracellular processing events via two orthogonal hyperspectral imaging approaches of near-infrared (NIR) fluorescence and resonance Raman scattering. A positive correlation between fluorescence and G-band intensities was uncovered within single cells, while exciton energy transfer and eventual aggregation of DNA-SWCNTs were observed to scale with increasing time after internalization. An analysis pipeline was developed to colocalize and deconvolute the fluorescence and Raman spectra of subcellular regions of interest (ROIs), allowing for single-chirality component spectra to be obtained with submicron spatial resolution. This approach uncovered correlations between DNA-SWCNT concentration, dielectric modulation, and irreversible aggregation within single intracellular vesicles. An immunofluorescence assay was designed to directly observe the DNA-SWCNTs in labeled endosomal vesicles, revealing a distinct relationship between the physical state of organelle-bound DNA-SWCNTs and the dynamic luminal conditions during endosomal maturation processes. Finally, we trained a machine learning algorithm to predict endosome type using the Raman spectra of the vesicle-bound DNA-SWCNTs, enabling major components in the endocytic pathway to be simultaneously visualized using a single intracellular reporter.

Disruption of a RAC1-centred network is associated with Alzheimer's disease pathology and causes age-dependent neurodegeneration

The molecular biological mechanisms of Alzheimer’s disease (AD) involve disease-associated crosstalk through many genes and include a loss of normal as well as a gain of abnormal interactions among genes. A protein domain network (PDN) is a collection of physical bindings that occur between protein domains, and the states of the PDNs in patients with AD are likely to be perturbed compared to those in normal healthy individuals. To identify PDN changes that cause neurodegeneration, we analysed the PDNs that occur among genes co-expressed in each of three brain regions at each stage of AD. Our analysis revealed that the PDNs collapsed with the progression of AD stage and identified five hub genes, including Rac1, as key players in PDN collapse. Using publicly available as well as our own gene expression data, we confirmed that the mRNA expression level of the RAC1 gene was downregulated in the entorhinal cortex (EC) of AD brains. To test the causality of these changes in neurodegeneration, we utilized Drosophila as a genetic model and found that modest knockdown of Rac1 in neurons was sufficient to cause age-dependent behavioural deficits and neurodegeneration. Finally, we identified a microRNA, hsa-miR-101-3p, as a potential regulator of RAC1 in AD brains. As the Braak neurofibrillary tangle (NFT) stage progressed, the expression levels of hsa-miR-101-3p were increased specifically in the EC. Furthermore, overexpression of hsa-miR-101-3p in the human neuronal cell line SH-SY5Y caused RAC1 downregulation. These results highlight the utility of our integrated network approach for identifying causal changes leading to neurodegeneration in AD.

Alix: A Candidate Serum Biomarker of Alzheimer's Disease

Alzheimer’s disease (AD) is the most common fatal neurodegenerative disease of the elderly worldwide. The identification of AD biomarkers will allow for earlier diagnosis and thus earlier intervention. The aim of this study was to find such biomarkers. It was observed that the expression of Alix was significantly decreased in brain tissues and serum samples from AD patients compared to the controls. A significant correlation between Alix levels and cognitive decline was observed (r = 0.80; p < 0.001) as well as a significant negative correlation between Alix and Aβ₄₀ in serum levels (r =−0.60, p < 0.001). The receiver operating characteristic curve (ROC) analysis showed the area under the curve (AUC) of Alix was 0.80, and the optimal cut-off point of 199.5 pg/ml was selected with the highest sum of sensitivity and specificity. The diagnostic accuracy for serum Alix was 74%, with 76% sensitivity and 71% specificity respectively, which could differentiate AD from controls. In addition, the expression of Alix was found to be significantly decreased in AD compared to vascular dementia (VaD). ROC analysis between AD and VaD showed that the AUC was 0.777, which could be indicative of the role of serum Alix as a biomarker in the differential diagnosis between AD and VaD. Most surprisingly, the decreased expression of Alix was attenuated after the treatment of Memantine in different AD animal models. In conclusion, our results indicate the possibility of serum Alix as a novel and non-invasive biomarker for AD for the first time.

Stress granules, RNA-binding proteins and polyglutamine diseases: too much aggregation?

Stress granules (SGs) are membraneless cell compartments formed in response to different stress stimuli, wherein translation factors, mRNAs, RNA-binding proteins (RBPs) and other proteins coalesce together. SGs assembly is crucial for cell survival, since SGs are implicated in the regulation of translation, mRNA storage and stabilization and cell signalling, during stress. One defining feature of SGs is their dynamism, as they are quickly assembled upon stress and then rapidly dispersed after the stress source is no longer present. Recently, SGs dynamics, their components and their functions have begun to be studied in the context of human diseases. Interestingly, the regulated protein self-assembly that mediates SG formation contrasts with the pathological protein aggregation that is a feature of several neurodegenerative diseases. In particular, aberrant protein coalescence is a key feature of polyglutamine (PolyQ) diseases, a group of nine disorders that are caused by an abnormal expansion of PolyQ tract-bearing proteins, which increases the propensity of those proteins to aggregate. Available data concerning the abnormal properties of the mutant PolyQ disease-causing proteins and their involvement in stress response dysregulation strongly suggests an important role for SGs in the pathogenesis of PolyQ disorders. This review aims at discussing the evidence supporting the existence of a link between SGs functionality and PolyQ disorders, by focusing on the biology of SGs and on the way it can be altered in a PolyQ disease context.

Amyloid-β oligomers block lysosomal targeting of miRNPs to prevent miRNP recycling and target repression in glial cells

Upon exposure to amyloid-β oligomers (Aβ1-42), glial cells start expressing proinflammatory cytokines, despite an increase in levels of repressive microRNAs (miRNAs). Exploring the mechanism of this potential immunity of target cytokine mRNAs against repressive miRNAs in amyloid-β-exposed glial cells, we have identified differential compartmentalization of repressive miRNAs in glial cells that explains this aberrant miRNA function. In Aβ1-42-treated cells, whereas target mRNAs were found to be associated with polysomes attached to endoplasmic reticulum (ER), the miRNA ribonucleoprotein complexes (miRNPs) were found to be present predominantly with endosomes that failed to recycle to ER-attached polysomes, preventing repression of mRNA targets. Aβ1-42 oligomers, by masking Rab7a proteins on endosomal surfaces, affected Rab7a interaction with Rab-interacting lysosomal protein (RILP), restricting the lysosomal targeting and recycling of miRNPs. RNA-processing body (P-body) localization of the miRNPs was found to be enhanced in amyloid-β-treated cells as a consequence of enhanced endosomal retention of miRNPs. Interestingly, depletion of P-body components partly rescued the miRNA function in glial cells exposed to amyloid-β and restricted the excess cytokine expression. This article has an associated First Person interview with the first author of the paper.

Clearance of intracellular tau protein from neuronal cells via VAMP8-induced secretion

In Alzheimer's disease (AD), tau, a microtubule-associated protein (MAP), becomes hyperphosphorylated, aggregates, and accumulates in the somato-dendritic compartment of neurons. In parallel to its intracellular accumulation in AD, tau is also released in the extracellular space, as revealed by its increased presence in cerebrospinal fluid (CSF). Consistent with this, recent studies, including ours, have reported that neurons secrete tau, and several therapeutic strategies aim to prevent the intracellular tau accumulation. Previously, we reported that late endosomes were implicated in tau secretion. Here, we explore the possibility of preventing intracellular tau accumulation by increasing tau secretion. Using neuronal models, we investigated whether overexpression of the vesicle-associated membrane protein 8 (VAMP8), an R-SNARE found on late endosomes, could increase tau secretion. The overexpression of VAMP8 significantly increased tau secretion, decreasing its intracellular levels in the neuroblastoma (N2a) cell line. Increased tau secretion by VAMP8 was also observed in murine hippocampal slices. The intracellular reduction of tau by VAMP8 overexpression correlated to a decrease of acetylated tubulin induced by tau overexpression in N2a cells. VAMP8 staining was preferentially found on late endosomes in N2a cells. Using total internal reflection fluorescence (TIRF) microscopy, the fusion of VAMP8-positive vesicles with the plasma membrane was correlated to the depletion of tau in the cytoplasm. Finally, overexpression of VAMP8 reduced the intracellular accumulation of tau mutants linked to frontotemporal dementia with parkinsonism and α-synuclein by increasing their secretion. Collectively, the present data indicate that VAMP8 could be used to increase tau and α-synuclein clearance to prevent their intracellular accumulation.

The industrial solvent trichloroethylene induces LRRK2 kinase activity and dopaminergic neurodegeneration in a rat model of Parkinson's disease

Gene-environment interaction is implicated in the majority of idiopathic Parkinson's disease (PD) risk, and some of the most widespread environmental contaminants are selectively toxic to dopaminergic neurons. Pesticides have long been connected to PD incidence, however, it has become increasingly apparent that other industrial byproducts likely influence neurodegeneration. For example, organic solvents, which are used in chemical, machining, and dry-cleaning industries, are of growing concern, as decades of solvent use and their effluence into the environment has contaminated much of the world's groundwater and soil. Like some pesticides, certain organic solvents, such as the chlorinated halocarbon trichloroethylene (TCE), are mitochondrial toxicants, which are collectively implicated in the pathogenesis of dopaminergic neurodegeneration. Recently, we hypothesized a possible gene-environment interaction may occur between environmental mitochondrial toxicants and the protein kinase LRRK2, mutations of which are the most common genetic cause of familial and sporadic PD. In addition, emerging data suggests that elevated wildtype LRRK2 kinase activity also contributes to the pathogenesis of idiopathic PD. To this end, we investigated whether chronic, systemic TCE exposure (200 mg/kg) in aged rats produced wildtype LRRK2 activation and caused nigrostriatal dopaminergic dysfunction. Interestingly, we found that TCE not only induced LRRK2 kinase activity in the brain, but produced a significant dopaminergic lesion in the nigrostriatal tract, elevated oxidative stress, and caused endolysosomal dysfunction and α-synuclein accumulation. Together, these data suggest that TCE-induced LRRK2 kinase activity contributed to the selective toxicity of dopaminergic neurons. We conclude that gene-environment interactions between certain industrial contaminants and LRRK2 likely influence PD risk.

A ketogenic diet differentially affects neuron and astrocyte transcription

Ketogenic diets (KDs) alter brain metabolism. Multiple mechanisms may account for their effects, and different brain regions may variably respond. Here, we considered how a KD affects brain neuron and astrocyte transcription. We placed male C57Bl6/N mice on either a 3-month KD or chow diet, generated enriched neuron and astrocyte fractions, and used RNA-Seq to assess transcription. Neurons from KD-treated mice generally showed transcriptional pathway activation while their astrocytes showed a mix of transcriptional pathway suppression and activation. The KD especially affected pathways implicated in mitochondrial and endoplasmic reticulum function, insulin signaling, and inflammation. An unbiased analysis of KD-associated expression changes strongly implicated transcriptional pathways altered in AD, which prompted us to explore in more detail the potential molecular relevance of a KD to AD. Our results indicate a KD differently affects neurons and astrocytes, and provide unbiased evidence that KD-induced brain effects are potentially relevant to neurodegenerative diseases such as AD.

Stress Response of Mouse Embryonic Fibroblasts Exposed to Polystyrene Nanoplastics

Polystyrene (PS) nanoplastic exposure has been shown to affect the viability of neuronal cells isolated from mouse embryonic brains. However, the viability of mouse embryonic fibroblasts (MEFs) was not affected although PS nanoplastics accumulated in the cytoplasm. It is currently unknown whether MEFs do not respond to PS nanoplastics or their cellular functions are altered without compromising viability. Here, we found that PS nanoplastics entered the cells via endocytosis and were then released into the cytoplasm, probably by endosomal escape, or otherwise remained in the endosome. Oxidative and inflammatory stress caused by intracellular PS nanoplastics induced the antioxidant response pathway and activated the autophagic pathway. However, colocalization of the autophagic marker LC3B and PS nanoplastics suggested that PS nanoplastics in the cytoplasm might interfere with normal autophagic function. Furthermore, autophagic flux could be impaired, probably due to accumulation of PS nanoplastic-containing lysosomes or autolysosomes. Intriguingly, the level of accumulated PS nanoplastics decreased during prolonged culture when MEFs were no longer exposed to PS nanoplastics. These results indicate that accumulated PS nanoplastics are removed or exported out of the cells. Therefore, PS nanoplastics in the cytoplasm affect cellular functions, but it is temporal and MEFs can overcome the stress caused by PS nanoplastic exposure.

Role of the endolysosomal pathway and exosome release in tau propagation

The progressive deposition of misfolded and aggregated forms of Tau protein in the brain is a pathological hallmark of tauopathies, such as Alzheimer's disease (AD) and frontotemporal degeneration (FTD). The misfolded Tau can be released into the extracellular space and internalized by neighboring cells, acting as seeds to trigger the robust conversion of soluble Tau into insoluble filamentous aggregates in a prion-like manner, ultimately contributing to the progression of the disease. However, molecular mechanisms accountable for the propagation of Tau pathology are poorly defined. We reviewed the Tau processing imbalance in endosomal, lysosomal, and exosomal pathways in AD. Increased exosome release counteracts the endosomal-lysosomal dysfunction of Tau processing but increases the number of aggregates and the propagation of Tau. This review summarizes our current understanding of the underlying tauopathy mechanisms with an emphasis on the emerging role of the endosomal-lysosomal-exosome pathways in this process. The components CHMP6, TSG101, and other components of the ESCRT complex, as well as Rab GTPase such as Rab35 and Rab7A, regulate vesicle cargoes routing from endosome to lysosome and affect Tau traffic, degradation, or secretion. Thus, the significant molecular pathways that should be potential therapeutic targets for treating tauopathies are determined.

Patient-derived iPSCs, a reliable in vitro model for the investigation of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease and a common cause of dementia among elderly individuals. The disease is characterized by progressive cognitive decline, accumulation of senile amyloid plaques and neurofibrillary tangles, oxidative stress, and inflammation. Human-derived cell models of AD are scarce, and over the years, non-human-derived models have been developed to recapitulate clinical AD, investigate the disease's pathogenesis and develop therapies for the disease. Several pharmacological compounds have been developed for AD based on findings from non-human-derived cell models; however, these pharmacological compounds have failed at different phases of clinical trials. This necessitates the application of human-derived cell models, such as induced pluripotent stem cells (iPSCs) in their optimized form in AD mechanistic studies and preclinical drug testing. This review provides an overview of AD and iPSCs. The AD-relevant phenotypes of iPSC-derived AD brain cells and the usefulness of iPSCs in AD are highlighted. Finally, the various recommendations that have been made to enhance iPSC/AD modelling are discussed.

Exosomes induce endolysosomal permeabilization as a gateway by which exosomal tau seeds escape into the cytosol

The microtubule-associated protein tau has a critical role in Alzheimer's disease and other tauopathies. A proposed pathomechanism in the progression of tauopathies is the trans-synaptic spreading of tau seeds, with a role for exosomes which are secretory nanovesicles generated by late endosomes. Our previous work demonstrated that brain-derived exosomes isolated from tau transgenic rTg4510 mice encapsulate tau seeds with the ability to induce tau aggregation in recipient cells. We had also shown that exosomes can hijack the endosomal pathway to spread through interconnected neurons. Here, we reveal how tau seeds contained within internalized exosomes exploit mechanisms of lysosomal degradation to escape the endosome and induce tau aggregation in the cytosol of HEK293T-derived 'tau biosensor cells'. We found that the majority of the exosome-containing endosomes fused with lysosomes to form endolysosomes. Exosomes induced their permeabilization, irrespective of the presence of tau seeds, or whether the exosomal preparations originated from mouse brains or HEK293T cells. We also found that permeabilization is a conserved mechanism, operating in both non-neuronal tau biosensor cells and primary neurons. However, permeabilization of endolysosomes only occurred in a small fraction of cells, which supports the notion that permeabilization occurs by a thresholded mechanism. Interestingly, tau aggregation was only induced in cells that exhibited permeabilization, presenting this as an escape route of exosomal tau seeds into the cytosol. Overexpression of RAB7, which is required for the formation of endolysosomes, strongly increased tau aggregation. Conversely, inhibition of lysosomal function with alkalinizing agents, or by knocking-down RAB7, decreased tau aggregation. Together, we conclude that the enzymatic activities of lysosomes permeabilize exosomal and endosomal membranes, thereby facilitating access of exosomal tau seeds to cytosolic tau to induce its aggregation. Our data underscore the importance of endosomal membrane integrity in mechanisms of cellular invasion by misfolded proteins that are resistant to lysosomal degradation.