Endosome Dysregulation in Down Syndrome: A Potential Contributor to Alzheimer Disease Pathology

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

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

Oxidative-Stress-Associated Proteostasis Disturbances and Increased DNA Damage in the Hippocampal Granule Cells of the Ts65Dn Model of Down Syndrome

Oxidative stress (OS) is one of the neuropathological mechanisms responsible for the deficits in cognition and neuronal function in Down syndrome (DS). The Ts65Dn (TS) mouse replicates multiple DS phenotypes including hippocampal-dependent learning and memory deficits and similar brain oxidative status. To better understand the hippocampal oxidative profile in the adult TS mouse, we analyzed cellular OS-associated alterations in hippocampal granule cells (GCs), a neuronal population that plays an important role in memory formation and that is particularly affected in DS. For this purpose, we used biochemical, molecular, immunohistochemical, and electron microscopy techniques. Our results indicate that TS GCs show important OS-associated alterations in the systems essential for neuronal homeostasis: DNA damage response and proteostasis, particularly of the proteasome and lysosomal system. Specifically, TS GCs showed: (i) increased DNA damage, (ii) reorganization of nuclear proteolytic factories accompanied by a decline in proteasome activity and cytoplasmic aggregation of ubiquitinated proteins, (iii) formation of lysosomal-related structures containing lipid droplets of cytotoxic peroxidation products, and (iv) mitochondrial ultrastructural defects. These alterations could be implicated in enhanced cellular senescence, accelerated aging and neurodegeneration, and the early development of Alzheimer's disease neuropathology present in TS mice and the DS population.

Improved mammalian retromer cryo-EM structures reveal a new assembly interface

Retromer (VPS26/VPS35/VPS29 subunits) assembles with multiple sorting nexin proteins on membranes to mediate endosomal recycling of transmembrane protein cargoes. Retromer has been implicated in other cellular processes, including mitochondrial homeostasis, nutrient sensing, autophagy, and fission events. Mechanisms for mammalian retromer assembly remain undefined, and retromer engages multiple sorting nexin proteins to sort cargoes to different destinations. Published structures demonstrate mammalian retromer forms oligomers in vitro, but several structures were poorly resolved. We report here improved retromer oligomer structures using single-particle cryo-EM by combining data collected from tilted specimens with multiple advancements in data processing, including using a 3D starting model for enhanced automated particle picking in RELION. We used a retromer mutant (3KE retromer) that breaks VPS35-mediated interfaces to determine a structure of a new assembly interface formed by the VPS26A and VPS35 N-termini. The interface reveals how an N-terminal VPS26A arrestin saddle can link retromer chains by engaging a neighboring VPS35 N- terminus, on the opposite side from the well-characterized C-VPS26/N-VPS35 interaction observed within heterotrimers. The new interaction interface exhibits substantial buried surface area (∼7000 Å²) and further suggests that metazoan retromer may serve as an adaptable scaffold.

Dysfunction of the retromer complex system contributes to amyloid and tau pathology in a stem cell model of Down syndrome

Introduction

Retromer complex proteins are decreased in Down syndrome (DS) brains and correlate inversely with brain amyloidosis. However, whether retromer dysfunction contributes to the amyloid beta (Aβ) and tau neuropathology of DS remains unknown.

Methods

Human trisomic induced Pluripotent Stem Cells (iPSCs) and isogenic controls were differentiated into forebrain neurons, and changes in retromer proteins, tau phosphorylated epitopes, and Aβ levels were assessed in euploid and trisomic neurons using western blot and enzyme-linked immunosorbent assay (ELISA). Genetic overexpression and pharmacological retromer stabilization were used to determine the functional role of the retromer complex system in modulating amyloid and tau pathology.

Results

Trisomic neurons developed age-dependent retromer core protein deficiency associated with accumulation of Aβ peptides and phosphorylated tau isoforms. Enhancing retromer function through overexpression or pharmacological retromer stabilization reduced amyloid and tau pathology in trisomic neurons. However, the effect was greater using a pharmacological approach, suggesting that targeting the complex stability may be more effective in addressing this neuropathology in DS.

Discussion

Our results demonstrate that the retromer complex is directly involved in the development of the neuropathologic phenotype in DS, and that pharmacological stabilization of the complex should be considered as a novel therapeutic tool in people with DS.

Rodent Modeling of Alzheimer's Disease in Down Syndrome: In vivo and ex vivo Approaches

There are an estimated 6 million people with Down syndrome (DS) worldwide. In developed countries, the vast majority of these individuals will develop Alzheimer's disease neuropathology characterized by the accumulation of amyloid-β (Aβ) plaques and tau neurofibrillary tangles within the brain, which leads to the early onset of dementia (AD-DS) and reduced life-expectancy. The mean age of onset of clinical dementia is ~55 years and by the age of 80, approaching 100% of individuals with DS will have a dementia diagnosis. DS is caused by trisomy of chromosome 21 (Hsa21) thus an additional copy of a gene(s) on the chromosome must cause the development of AD neuropathology and dementia. Indeed, triplication of the gene APP which encodes the amyloid precursor protein is sufficient and necessary for early onset AD (EOAD), both in people who have and do not have DS. However, triplication of other genes on Hsa21 leads to profound differences in neurodevelopment resulting in intellectual disability, elevated incidence of epilepsy and perturbations to the immune system. This different biology may impact on how AD neuropathology and dementia develops in people who have DS. Indeed, genes on Hsa21 other than APP when in three-copies can modulate AD-pathogenesis in mouse preclinical models. Understanding this biology better is critical to inform drug selection for AD prevention and therapy trials for people who have DS. Here we will review rodent preclinical models of AD-DS and how these can be used for both in vivo and ex vivo (cultured cells and organotypic slice cultures) studies to understand the mechanisms that contribute to the early development of AD in people who have DS and test the utility of treatments to prevent or delay the development of disease.

Whole-Transcriptome Analysis of Serum L1CAM-Captured Extracellular Vesicles Reveals Neural and Glycosylation Changes in Autism Spectrum Disorder

The pathophysiology of autistic spectrum disorder (ASD) is not fully understood and there are no diagnostic or predictive biomarkers. Extracellular vesicles (EVs) are cell-derived nano-sized vesicles, carrying nucleic acids, proteins, lipids, and other bioactive substances. As reported, serum neural cell adhesion molecule L1 (L1CAM)-captured EVs (LCEVs) can provide reliable biomarkers for neurological diseases; however, little is known about the LCEVs in children with ASD. The study enrolled 100 children with ASD (2.5-6 years of age; 90 males) and 60 age-matched TD children (54 males) as control. The serum sample was collected and pooled into five ASD subgroups and three TD subgroups (n = 20). LCEVs were isolated and characterized meticulously. Whole-transcriptome of LCEVs was analyzed by lncRNA microarray and RNA-sequencing. All raw data was submitted on GEO Profiles, and GEO accession numbers is GSE186493. RNAs expressed differently in LCEVs from ASD sera vs. TD sera were screened, analyzed, and further validated. A total of 1418 mRNAs, 1745 lncRNAs, and 11 miRNAs were differentially expressed, and most of them were downregulated in ASD. Most RNAs were involved in neuron- and glycan-related networks implicated in ASD. The levels of EDNRA, SLC17A6, HTR3A, OSTC, TMEM165, PC-5p-139289_26, and hsa-miR-193a-5p were validated in at least 15 ASD and 15 TD individual serum samples, which were consistent with the results of transcriptome analysis. In conclusion, whole-transcriptome analysis of serum LCEVs reveals neural and glycosylation changes in ASD, which may help detect predictive biomarkers and molecular mechanisms of ASD, and provide reference for diagnoses and therapeutic management of the disease.

The association of retromer deficiency and tau pathology in Down syndrome

Retromer deficiency is reported in Down syndrome and correlates with amyloidosis, however, its association with tau neuropathology remains unclear. Down syndrome and control brain tissues were evaluated for phosphorylated tau, tau modulators, and cathepsin-D activity. Several kinases and phosphatase PP2A were unchanged, but tau phosphorylation was elevated, and cathepsin-D activity decreased in aged Down syndrome subjects. Retromer proteins positively associated with soluble tau, whereas pathogenic tau negatively correlated with retromer proteins and cathepsin-D activity. Retromer deficiency and consequent reduction of cathepsin-D activity may contribute to pathogenic tau accumulation, thus, retromer represents a viable therapeutic target against tau pathology in Down syndrome. This article is protected by copyright. All rights reserved.

Dysregulation of the Retromer Complex in Brain Endothelial Cells Results in Accumulation of Phosphorylated Tau

Introduction

Transport through endothelial cells of the blood–brain barrier (BBB) involves a complex group of structures of the endo-lysosome system such as early and late endosomes, and the retromer complex system. Studies show that neuronal dysregulation of the vacuolar protein sorting 35 (VPS35), the main component of the retromer complex recognition core, results in altered protein trafficking and degradation and is involved in neurodegeneration. Since the functional role of VPS35 in endothelial cells has not been fully investigated, in the present study we aimed at characterizing the effect of its downregulation on these pathways.

Methods

Genetic silencing of VPS35 in human brain endothelial cells; measurement of retromer complex system proteins, autophagy and ubiquitin-proteasome systems.

Results

VPS35-downregulated endothelial cells had increased expression of LC3B2/1 and more ubiquitinated products, markers of autophagy flux and impaired proteasome activity, respectively. Additionally, compared with controls VPS35 downregulation resulted in significant accumulation of tau protein and its phosphorylated isoforms.

Discussion

Our findings demonstrate that in brain endothelial cells retromer complex dysfunction by influencing endosome-lysosome degradation pathways results in altered proteostasis. Restoration of the retromer complex system function should be considered a novel therapeutic approach to rescue endothelial protein transport.

Endosomal Recycling Defects and Neurodevelopmental Disorders

The quality and quantity of membrane proteins are precisely and dynamically maintained through an endosomal recycling process. This endosomal recycling is executed by two protein complexes: retromer and recently identified retriever. Defects in the function of retromer or retriever cause dysregulation of many membrane proteins and result in several human disorders, including neurodegenerative disorders such as Alzheimer’s disease and Parkinson’s disease. Recently, neurodevelopmental disorders caused by pathogenic variants in genes associated with retriever were identified. This review focuses on the two recycling complexes and discuss their biological and developmental roles and the consequences of defects in endosomal recycling, especially in the nervous system. We also discuss future perspectives of a possible relationship of the dysfunction of retromer and retriever with neurodevelopmental disorders.

VPS35 Downregulation Alters Degradation Pathways in Neuronal Cells

BACKGROUND

The vacuolar protein sorting 35 (VPS35) is the main component of the retromer recognition core complex system which regulates intracellular cargo protein sorting and trafficking. Downregulation of VPS35 has been linked to the pathogenesis of neurodegenerative disorders such Alzheimer's and Parkinson's diseases via endosome dysregulation.

OBJECTIVE

Here we show that the genetic manipulation of VPS35 affects intracellular degradation pathways.

METHODS

A neuronal cell line expressing human APP Swedish mutant was used. VPS35 silencing was performed treating cells with VPS35 siRNA or Ctr siRNA for 72 h.

RESULTS

Downregulation of VPS35 was associated with alteration of autophagy flux and intracellular accumulation of acidic and ubiquitinated aggregates suggesting that dysfunction of the retromer recognition core leads to a significant alteration in both pathways.

CONCLUSION

Taken together, our data demonstrate that besides cargo sorting and trafficking, VPS35 by supporting the integral function of the retromer complex system plays an important role also as a critical regulator of intracellular degradation pathways.