Endocytic disturbances distinguish among subtypes of alzheimer's disease and related disorders

Wiskostatin and other carbazole scaffolds as off target inhibitors of dynamin I GTPase activity and endocytosis

Wiskostatin (1-(3,6-dibromo-9H-carbazol-9-yl)-3-(dimethylamino)propan-2-ol) (1) is a carbazole-based compound reported as a specific and relatively potent inhibitor of the N-WASP actin remodelling complex (S-isomer EC₅₀ = 4.35 μM; R-isomer EC₅₀ = 3.44 μM). An NMR solution structure showed that wiskostatin interacts with a cleft in the regulatory GTPase binding domain of N-WASP. However, numerous studies have reported wiskostatin's actions on membrane transport and cytokinesis that are independent of the N-WASP-Arp2/3 complex pathway, but offer limited alternative explanation. The large GTPase, dynamin has established functional roles in these pathways. This study reveals that wiskostatin and its analogues, as well as other carbazole-based compounds, are inhibitors of helical dynamin GTPase activity and endocytosis. We characterise the effects of wiskostatin on in vitro dynamin GTPase activity, in-cell endocytosis, and determine the importance of wiskostatin functional groups on these activities through design and synthesis of libraries of wiskostatin analogues. We also examine whether other carbazole-based scaffolds frequently used in research or the clinic also modulate dynamin and endocytosis. Understanding off-targets for compounds used as research tools is important to be able to confidently interpret their action on biological systems, particularly when the target and off-targets affect overlapping mechanisms (e.g. cytokinesis and endocytosis). Herein we demonstrate that wiskostatin is a dynamin inhibitor (IC₅₀ 20.7 ± 1.2 μM) and a potent inhibitor of clathrin mediated endocytosis (IC₅₀ = 6.9 ± 0.3 μM). Synthesis of wiskostatin analogues gave rise to 1-(9H-carbazol-9-yl)-3-((4-methylbenzyl)amino)propan-2-ol (35) and 1-(9H-carbazol-9-yl)-3-((4-chlorobenzyl)amino)propan-2-ol (43) as potent dynamin inhibitors (IC₅₀ = 1.0 ± 0.2 μM), and (S)-1-(3,6-dibromo-9H-carbazol-9-yl)-3-(dimethylamino)propan-2-ol (8a) and (R)-1-(3,6-dibromo-9H-carbazol-9-yl)-3-(dimethylamino)propan-2-ol (8b) that are amongst the most potent inhibitors of clathrin mediated endocytosis yet reported (IC₅₀ = 2.3 ± 3.3 and 2.1 ± 1.7 μM, respectively).

Exosome-based approaches in the management of Alzheimer's disease

Alzheimer's disease (AD) has been the most extensively studied neurological disorders that affects millions of individuals globally and is associated with misfolding of proteins in the brain. Amyloid-β and tau are predominantly involved in the pathogenesis of AD. Therapeutic interventions and nanotechnological advancements are useful only in managing the AD symptoms and the cure for this disease remains elusive. Exosomes, originating from most cell and tissue types are regarded as a double-edged sword, considering their roles in the progression and treatment of AD. Exosomes can be manipulated as drug delivery vehicles for a wide range of therapeutic cargos-both small molecules and macromolecules. Herein, we review the roles of exosomes in the pathology, diagnosis, and treatment of AD and highlight their application as a drug carrier to the brain for AD treatment.

Endosomal Sorting Protein SNX27 and Its Emerging Roles in Human Cancers

SNX27 belongs to the sorting nexin (SNX) family of proteins that play a critical role in protein sorting and trafficking in the endocytosis pathway. This protein family is characterized by the presence of a Phox (PX) domain; however, SNX27 is unique in containing an additional PDZ domain. Recently, SNX27 has gained popularity as an important sorting protein that is associated with the retromer complex and mediates the recycling of internalized proteins from endosomes to the plasma membrane in a PDZ domain-dependent manner. Over 100 cell surface proteins have been identified as binding partners of the SNX27-retromer complex. However, the roles and underlying mechanisms governed by SNX27 in tumorigenesis remains to be poorly understood. Many of its known binding partners include several G-protein coupled receptors, such as β2-andrenergic receptor and parathyroid hormone receptor, are associated with multiple pathways implicated in oncogenic signaling and tumorigenesis. Additionally, SNX27 mediates the recycling of GLUT1 and the activation of mTORC1, both of which can regulate intracellular energy balance and promote cell survival and proliferation under conditions of nutrient deprivation. In this review, we summarize the structure and fundamental roles of SNX proteins, with a focus on SNX27, and provide the current evidence indicating towards the role of SNX27 in human cancers. We also discuss the gap in the field and future direction of SNX27 research. Insights into the emerging roles and mechanism of SNX27 in cancers will provide better development strategies to prevent and treat tumorigenesis.

Finding memo: versatile interactions of the VPS10p-Domain receptors in Alzheimer’s disease

The family of VPS10p-Domain (D) receptors comprises five members named SorLA, Sortilin, SorCS1, SorCS2 and SorCS3. While their physiological roles remain incompletely resolved, they have been recognized for their signaling engagements and trafficking abilities, navigating a number of molecules between endosome, Golgi compartments, and the cell surface. Strikingly, recent studies connected all the VPS10p-D receptors to Alzheimer’s disease (AD) development. In addition, they have been also associated with diseases comorbid with AD such as diabetes mellitus and major depressive disorder. This systematic review elaborates on genetic, functional, and mechanistic insights into how dysfunction in VPS10p-D receptors may contribute to AD etiology, AD onset diversity, and AD comorbidities. Starting with their functions in controlling cellular trafficking of amyloid precursor protein and the metabolism of the amyloid beta peptide, we present and exemplify how these receptors, despite being structurally similar, regulate various and distinct cellular events involved in AD. This includes a plethora of signaling crosstalks that impact on neuronal survival, neuronal wiring, neuronal polarity, and synaptic plasticity. Signaling activities of the VPS10p-D receptors are especially linked, but not limited to, the regulation of neuronal fitness and apoptosis via their physical interaction with pro- and mature neurotrophins and their receptors. By compiling the functional versatility of VPS10p-D receptors and their interactions with AD-related pathways, we aim to further propel the AD research towards VPS10p-D receptor family, knowledge that may lead to new diagnostic markers and therapeutic strategies for AD patients.

Linking glycosphingolipids to Alzheimer's amyloid-ß: extracellular vesicles and functional plant materials

Glycosphingolipids (GSLs) are a specialized class of membrane lipids composed of a ceramide and a carbohydrate head group. GSLs are localized in cell membranes and were recently found to be enriched in the membrane of neuron-derived exosomes, which are a type of extracellular vesicle. Our studies demonstrated that exosomal GSLs may be associated with the amyloid-ß (Aß) peptide, a principal agent of Alzheimer's disease (AD), and act to clear Aß by transporting Aß into brain phagocytic microglia. In this review, we summarize and discuss the function of exosomal GSLs in Aß homeostasis in AD pathology. Improvement in Aß clearance is a potent strategy for AD prevention and therapy. Dietary glucosylceramides (GlcCer) isolated from plants are absorbed into the body as various metabolites, including ceramides. Our recent work demonstrated that dietary GlcCer accelerates neuronal exosome production, which facilitates Aß clearance in mice. Furthermore, studies of AD model mice and human clinical trials have found that oral administration of plant-type GlcCer attenuates the Aß burden in the brain. We also introduce the development of plant-type GlcCer as functional food materials to prevent AD.

Pyrimidyn-Based Dynamin Inhibitors as Novel Cytotoxic Agents

Five focused libraries of pyrimidine-based dynamin GTPase inhibitors, in total 69 compounds were synthesised, and their dynamin inhibition and broad-spectrum cytotoxicity examined. Dynamin plays a crucial role in mitosis, and as such inhibition of dynamin was expected to broadly correlate with the observed cytotoxicity. The pyrimidines synthesised ranged from mono-substituted to trisubstituted. The highest levels of dynamin inhibition were noted with di- and tri- substituted pyrimidines, especially those with pendent amino alkyl chains. Short chains and simple heterocyclic rings reduced dynamin activity. There were three levels of dynamin activity noted: 1-10, 10-25 and 25-60 μM. Screening of these compounds in a panel of cancer cell lines: SW480 (colon), HT29 (colon), SMA (spontaneous murine astrocytoma), MCF-7 (breast), BE2-C (glioblastoma), SJ-G2 (neuroblastoma), MIA (pancreas), A2780 (ovarian), A431 (skin), H460 (lung), U87 (glioblastoma) and DU145 (prostate) cell lines reveal a good correlation between the observed dynamin inhibition and the observed cytotoxicity. The most active analogues (31 a,b) developed returned average GI₅₀ values of 1.0 and 0.78 μM across the twelve cell lines examined. These active analogues were: N² -(3-dimethylaminopropyl)-N⁴ -dodecyl-6-methylpyrimidine-2,4-diamine (31 a) and N⁴ -(3-dimethylaminopropyl)-N² -dodecyl-6-methylpyrimidine-2,4-diamine (31 b).

Rare variants in the endocytic pathway are associated with Alzheimer's disease, its related phenotypes, and functional consequences

Late-onset Alzheimer's disease (LOAD) is the most common type of dementia causing irreversible brain damage to the elderly and presents a major public health challenge. Clinical research and genome-wide association studies have suggested a potential contribution of the endocytic pathway to AD, with an emphasis on common loci. However, the contribution of rare variants in this pathway to AD has not been thoroughly investigated. In this study, we focused on the effect of rare variants on AD by first applying a rare-variant gene-set burden analysis using genes in the endocytic pathway on over 3,000 individuals with European ancestry from three large whole-genome sequencing (WGS) studies. We identified significant associations of rare-variant burden within the endocytic pathway with AD, which were successfully replicated in independent datasets. We further demonstrated that this endocytic rare-variant enrichment is associated with neurofibrillary tangles (NFTs) and age-related phenotypes, increasing the risk of obtaining severer brain damage, earlier age-at-onset, and earlier age-of-death. Next, by aggregating rare variants within each gene, we sought to identify single endocytic genes associated with AD and NFTs. Careful examination using NFTs revealed one significantly associated gene, ANKRD13D. To identify functional associations, we integrated bulk RNA-Seq data from over 600 brain tissues and found two endocytic expression genes (eGenes), HLA-A and SLC26A7, that displayed significant influences on their gene expressions. Differential expressions between AD patients and controls of these three identified genes were further examined by incorporating scRNA-Seq data from 48 post-mortem brain samples and demonstrated distinct expression patterns across cell types. Taken together, our results demonstrated strong rare-variant effect in the endocytic pathway on AD risk and progression and functional effect of gene expression alteration in both bulk and single-cell resolution, which may bring more insight and serve as valuable resources for future AD genetic studies, clinical research, and therapeutic targeting.

Ultrastructural and dynamic studies of the endosomal compartment in Down syndrome

Enlarged early endosomes have been visualized in Alzheimer's disease (AD) and Down syndrome (DS) using conventional confocal microscopy at a resolution corresponding to endosomal size (hundreds of nm). In order to overtake the diffraction limit, we used super-resolution structured illumination microscopy (SR-SIM) and transmission electron microscopies (TEM) to analyze the early endosomal compartment in DS.By immunofluorescence and confocal microscopy, we confirmed that the volume of Early Endosome Antigen 1 (EEA1)-positive puncta was 13-19% larger in fibroblasts and iPSC-derived neurons from individuals with DS, and in basal forebrain cholinergic neurons (BFCN) of the Ts65Dn mice modelling DS. However, EEA1-positive structures imaged by TEM or SR-SIM after chemical fixation had a normal size but appeared clustered. In order to disentangle these discrepancies, we imaged optimally preserved High Pressure Freezing (HPF)-vitrified DS fibroblasts by TEM and found that early endosomes were 75% denser but remained normal-sized.RNA sequencing of DS and euploid fibroblasts revealed a subgroup of differentially-expressed genes related to cargo sorting at multivesicular bodies (MVBs). We thus studied the dynamics of endocytosis, recycling and MVB-dependent degradation in DS fibroblasts. We found no change in endocytosis, increased recycling and delayed degradation, suggesting a "traffic jam" in the endosomal compartment.Finally, we show that the phosphoinositide PI (3) P, involved in early endosome fusion, is decreased in DS fibroblasts, unveiling a new mechanism for endosomal dysfunctions in DS and a target for pharmacotherapy.

Down syndrome

Trisomy 21, the presence of a supernumerary chromosome 21, results in a collection of clinical features commonly known as Down syndrome (DS). DS is among the most genetically complex of the conditions that are compatible with human survival post-term, and the most frequent survivable autosomal aneuploidy. Mouse models of DS, involving trisomy of all or part of human chromosome 21 or orthologous mouse genomic regions, are providing valuable insights into the contribution of triplicated genes or groups of genes to the many clinical manifestations in DS. This endeavour is challenging, as there are >200 protein-coding genes on chromosome 21 and they can have direct and indirect effects on homeostasis in cells, tissues, organs and systems. Although this complexity poses formidable challenges to understanding the underlying molecular basis for each of the many clinical features of DS, it also provides opportunities for improving understanding of genetic mechanisms underlying the development and function of many cell types, tissues, organs and systems. Since the first description of trisomy 21, we have learned much about intellectual disability and genetic risk factors for congenital heart disease. The lower occurrence of solid tumours in individuals with DS supports the identification of chromosome 21 genes that protect against cancer when overexpressed. The universal occurrence of the histopathology of Alzheimer disease and the high prevalence of dementia in DS are providing insights into the pathology and treatment of Alzheimer disease. Clinical trials to ameliorate intellectual disability in DS signal a new era in which therapeutic interventions based on knowledge of the molecular pathophysiology of DS can now be explored; these efforts provide reasonable hope for the future.

Three-dimensional modeling of human neurodegeneration: brain organoids coming of age

The prevalence of dementia and other neurodegenerative diseases is rapidly increasing in aging nations. These relentless and progressive diseases remain largely without disease-modifying treatments despite decades of research and investments. It is becoming clear that traditional two-dimensional culture and animal model systems, while providing valuable insights on the major pathophysiological pathways associated with these diseases, have not translated well to patients' bedside. Fortunately, the advent of induced-pluripotent stem cells and three-dimensional cell culture now provide tools that are revolutionizing the study of human diseases by permitting analysis of patient-derived human tissue with non-invasive procedures. Specifically, brain organoids, self-organizing neural structures that can mimic human fetal brain development, have now been harnessed to develop alternative models of Alzheimer's disease, Parkinson's disease, motor neuron disease, and Frontotemporal dementia by recapitulating important neuropathological hallmarks found in these disorders. Despite these early breakthroughs, several limitations need to be vetted in brain organoid models in order to more faithfully match human tissue qualities, including relative tissue immaturity, lack of vascularization and incomplete cellular diversity found in this culture system. Here, we review current brain organoid protocols, the pathophysiology of neurodegenerative disorders, and early studies with brain organoid neurodegeneration models. We then discuss the multiple engineering and conceptual challenges surrounding their use and provide possible solutions and exciting avenues to be pursued. Altogether, we believe that brain organoids models, improved with classical and emerging molecular and analytic tools, have the potential to unravel the opaque pathophysiological mechanisms of neurodegeneration and devise novel treatments for an array of neurodegenerative disorders.

Altered γ-Secretase Processing of APP Disrupts Lysosome and Autophagosome Function in Monogenic Alzheimer's Disease

Abnormalities of the endolysosomal and autophagy systems are found in Alzheimer’s disease, but it is not clear whether defects in these systems are a cause or consequence of degenerative processes in the disease. In human neuronal models of monogenic Alzheimer’s disease, APP and PSEN1 mutations disrupt lysosome function and autophagy, leading to impaired lysosomal proteolysis and defective autophagosome clearance. Processing of APP by γ-secretase is central to the pathogenic changes in the lysosome-autophagy system caused by PSEN1 and APP mutations: reducing production of C-terminal APP by inhibition of BACE1 rescued these phenotypes in both APP and PSEN1 mutant neurons, whereas inhibition of γ-secretase induced lysosomal and autophagic pathology in healthy neurons. Defects in lysosomes and autophagy due to PSEN1 mutations are rescued by CRISPR-knockout of APP. These data demonstrate a key role for proteolysis of the C-terminal of APP by γ-secretase in neuronal dysfunction in monogenic Alzheimer’s disease.

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APP and PSEN1 mutant neurons have deficits in lysosome proteolysis

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BACE1 inhibition rescues lysosome and autophagy defects

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PSEN1 mutant phenotypes are rescued by genetic deletion of APP

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Lysosome and autophagy defects are causes of neuronal dysfunction in AD

Back to the origins: Human brain organoids to investigate neurodegeneration

Neurodegenerative disorders represent a high burden in terms of individual, social and economical resources. No ultimate therapy has been established so far; human brain morphology and development can not be entirely reproduced by animal models, and genomic, metabolic and biochemical differences might contribute to a limited predictive power for human translation. Thus, the development of human brain organoid models holds a wide potential to investigate the range of physiological and pathological features that characterise the early onset of the degeneration. Moreover, central nervous system development has gained a crucial role in the study of the pathogenesis of neurodegenerative disorders. Premature alterations during brain maturation have been related to late disease manifestations; genetic mutations responsible for neurodegeneration have been found in genes highly expressed during neural development. Elucidating the mechanisms triggering neuronal susceptibility to degeneration is crucial for pathogenetic studies and therapeutic discoveries. In the present work, we provide an overview on the current applications of human brain organoids towards studies of neurodegenerative diseases, with a survey on the recent discoveries and a closing discussion on the present challenges and future perspectives.

Focusing on cellular biomarkers: The endo-lysosomal pathway in Down syndrome

Down syndrome (DS) is the most frequent chromosomal disorder. It is caused by the triplication of human chromosome 21, leading to increased dosage of a variety of genes including APP (Amyloid Precursor Protein). Mainly for this reason, individuals with DS are at high risk to develop Alzheimer's disease (AD). Extensive literature identified various morphological and molecular abnormalities in the endo-lysosomal pathway both in DS and AD. Most studies in this field investigated the causative role of APP (Amyloid Precursor Protein) in endo-lysosomal dysfunctions, thus linking phenotypes observed in DS and AD. In DS context, several lines of evidence and emerging hypotheses suggest that other molecular players and pathways may be implicated in these complex phenotypes. In this review, we outline the normal functioning of endosomal trafficking and summarize the research on endo-lysosomal dysfunction in DS in light of AD findings. We emphasize the role of genes of chromosome 21 implicated in endocytosis to explain endosomal abnormalities and set the limitations and perspectives of models used to explore endo-lysosomal dysfunction in DS and find new biomarkers. The review highlights the complexity of endo-lysosomal dysfunction in DS and suggests directions for future research in the field.

Alzheimer Disease Pathogenesis: Insights From Molecular and Cellular Biology Studies of Oligomeric Aβ and Tau Species

Alzheimer disease (AD) represents an oncoming epidemic that without an effective treatment promises to exact extraordinary human and financial burdens. Studies of pathogenesis are essential for defining targets for discovering disease-modifying treatments. Past studies of AD neuropathology provided valuable, albeit limited, insights. Nevertheless, building on these findings, recent studies have provided an increasingly rich harvest of genetic, molecular and cellular data that are creating unprecedented opportunities to both understand and treat AD. Among the most significant are those documenting the presence within the AD brain of toxic oligomeric species of Aβ and tau. Existing data support the view that such species can propagate and spread within neural circuits. To place these findings in context we first review the genetics and neuropathology of AD, including AD in Down syndrome (AD-DS). We detail studies that support the existence of toxic oligomeric species while noting the significant unanswered questions concerning their precise structures, the means by which they spread and undergo amplification and how they induce neuronal dysfunction and degeneration. We conclude by offering a speculative synthesis for how oligomers of Aβ and tau initiate and drive pathogenesis. While 100 years after Alzheimer's first report there is much still to learn about pathogenesis and the discovery of disease-modifying treatments, the application of new concepts and sophisticated new tools are poised to deliver important advances for combatting AD.

Assessing the role of Porphyromonas gingivalis in periodontitis to determine a causative relationship with Alzheimer's disease

Chronic periodontitis of 10 years' duration is reported to become a twofold risk factor for the development of Alzheimer's disease (AD). Periodontitis is modifiable, and this fits with the current action plan for preventing AD. However, until periodontitis, becomes acknowledged as a firm risk factor for AD, this risk will continue. Here, we put forward our own argument based on the current literature for in vivo infection-mediated periodontal disease models supporting the antimicrobial protection hypothesis of AD and interventional studies supporting the causal links. Oral infections with Porphyromonas gingivalis, or introduction of its lipopolysaccharide (LPS), in various mouse models has demonstrated the development of key neuropathological hallmark lesions defining AD. These are extracellular amyloid-beta plaques, phosphorylated tau, neurofibrillary tangles, widespread acute and chronic inflammation, blood-brain barrier defects together with the clinical phenotype showing impaired learning and spatial memory. Live P. gingivalis and its LPS (commercial or from 'microbullets') are powerful peripheral and intracerebral inflammatory signalling initiators, and this has direct implications on memory and lesion development. Maintaining a healthy oral microbiome and managing periodontal disease with regular surveillance and good oral hygiene throughout life is likely to reduce the unnecessary burden of AD in some individuals.

Cystatin C prevents neuronal loss and behavioral deficits via the endosomal pathway in a mouse model of down syndrome

Cystatin C (CysC) plays diverse protective roles under conditions of neuronal challenge. We investigated whether CysC protects from trisomy-induced pathologies in a mouse model of Down syndrome (DS), the most common cause of developmental cognitive and behavioral impairments in humans. We have previously shown that the segmental trisomy mouse model, Ts[Rb(12.1716)]2Cje (Ts2) has DS-like neuronal and behavioral deficiencies. The current study reveals that transgene-mediated low levels of human CysC overexpression has a preventive effect on numerous neuropathologies in the brains of Ts2 mice, including reducing early and late endosome enlargement in cortical neurons and decreasing loss of basal forebrain cholinergic neurons (BFCNs). Consistent with these cellular benefits, behavioral dysfunctions were also prevented, including deficits in nesting behavior and spatial memory. We determined that the CysC-induced neuroprotective mechanism involves activation of the phosphotidylinositol kinase (PI3K)/AKT pathway. Activating this pathway leads to enhanced clearance of accumulated endosomal substrates, protecting cells from DS-mediated dysfunctions in the endosomal system and, for BFCNs, from neurodegeneration. Our findings suggest that modulation of the PI3/AKT pathway offers novel therapeutic interventions for patients with DS.

The Retromer Complex and Sorting Nexins in Neurodegenerative Diseases

The retromer complex and associated sorting nexins (SNXs) comprise a critical trafficking machinery which mediates endosomal protein sorting. Retromer and/or SNX dysfunction has been linked to several neurodegenerative diseases including Alzheimer’s disease (AD), Parkinson’s disease (PD), and Down’s syndrome (DS). In AD, deficiency of the retromer complex or its cargo proteins impairs endosomal trafficking of amyloid precursor protein (APP), resulting in the overproduction of β-amyloid (Aβ). Several SNX components directly interact with APP or APP-cleaving enzymes (β- and γ-secretases) to regulate amyloidogenic APP processing and Aβ generation. In addition, PD-linked mutations in retromer components cause mistrafficking of retromer cargo proteins and mitochondrial dysfunction, and dysregulation retromer-mediated trafficking has been considered as an important cause of hereditary spastic paraplegia (HSP) and neuronal ceroid lipofuscinoses (NCLs). Moreover, SNX27 deficiency is an important contributor for synaptic and cognitive impairment in DS. Here we review recent findings describing the retromer complex and/or SNXs-mediated endosomal sorting in neurodegenerative disorders.

Dysregulation of neurotrophin signaling in the pathogenesis of Alzheimer disease and of Alzheimer disease in Down syndrome

Neurotrophic factors, including the members of the neurotrophin family, play important roles in the development and maintenance of the nervous system. Trophic factor signals must be transmitted over long distances from axons and dendrites to the cell bodies of neurons. A mode of signaling well suited to the challenge of robust long distance signaling is the signaling endosome. We review the biology of signaling endosomes and the "signaling endosome hypothesis". Evidence for disruption of signaling endosome function in disorders of the nervous system is also reviewed. Changes in endosome structure in Alzheimer disease (AD) and Down syndrome (DS) are present early in these disorders. Data for the APP products responsible are reviewed and the consequent changes in signaling from endosomes discussed. We conclude by pointing to the need for additional studies to explore the biology of signaling endosomes in normal neurons and to elucidate their role in the pathogenesis of neurodegeneration.

Stem Cell Technology for (Epi)genetic Brain Disorders

Despite the enormous efforts of the scientific community over the years, effective therapeutics for many (epi)genetic brain disorders remain unidentified. The common and persistent failures to translate preclinical findings into clinical success are partially attributed to the limited efficiency of current disease models. Although animal and cellular models have substantially improved our knowledge of the pathological processes involved in these disorders, human brain research has generally been hampered by a lack of satisfactory humanized model systems. This, together with our incomplete knowledge of the multifactorial causes in the majority of these disorders, as well as a thorough understanding of associated (epi)genetic alterations, has been impeding progress in gaining more mechanistic insights from translational studies. Over the last years, however, stem cell technology has been offering an alternative approach to study and treat human brain disorders. Owing to this technology, we are now able to obtain a theoretically inexhaustible source of human neural cells and precursors in vitro that offer a platform for disease modeling and the establishment of therapeutic interventions. In addition to the potential to increase our general understanding of how (epi)genetic alterations contribute to the pathology of brain disorders, stem cells and derivatives allow for high-throughput drugs and toxicity testing, and provide a cell source for transplant therapies in regenerative medicine. In the current chapter, we will demonstrate the validity of human stem cell-based models and address the utility of other stem cell-based applications for several human brain disorders with multifactorial and (epi)genetic bases, including Parkinson's disease (PD), Alzheimer's disease (AD), fragile X syndrome (FXS), Angelman syndrome (AS), Prader-Willi syndrome (PWS), and Rett syndrome (RTT).

Lysosomal dysfunction in the brain of a mouse model with intraneuronal accumulation of carboxyl terminal fragments of the amyloid precursor protein

Recent data suggest that intraneuronal accumulation of metabolites of the amyloid-β-precursor protein (APP) is neurotoxic. We observed that transgenic mice overexpressing in neurons a human APP gene harboring the APP^E693Q (Dutch) mutation have intraneuronal lysosomal accumulation of APP carboxylterminal fragments (APP-CTFs) and oligomeric amyloid β (oAβ) but no histological evidence of amyloid deposition. Morphometric quantification using the lysosomal marker protein 2 (LAMP-2) immunolabeling showed higher neuronal lysosomal counts in brain of 12-months-old APP^E693Q as compared with age-matched non-transgenic littermates, and western blots showed increased lysosomal proteins including LAMP-2, cathepsin D and LC3. At 24 months of age, these mice also exhibited an accumulation of α-synuclein in the brain, along with increased conversion of LC3-I to LC3-II, an autophagosomal/autolysosomal marker. In addition to lysosomal changes at 12 months of age, these mice developed cholinergic neuronal loss in the basal forebrain, GABAergic neuronal loss in the cortex, hippocampus and basal forebrain and gliosis and microgliosis in the hippocampus. These findings suggest a role for the intraneuronal accumulation of oAβ and APP-CTFs and resultant lysosomal pathology at early stages of Alzheimer's disease-related pathology.

Modeling neurodegenerative diseases with patient-derived induced pluripotent cells: Possibilities and challenges

The rising prevalence of progressive neurodegenerative diseases coupled with increasing longevity poses an economic burden at individual and societal levels. There is currently no effective cure for the majority of neurodegenerative diseases and disease-affected tissues from patients have been difficult to obtain for research and drug discovery in pre-clinical settings. While the use of animal models has contributed invaluable mechanistic insights and potential therapeutic targets, the translational value of animal models could be further enhanced when combined with in vitro models derived from patient-specific induced pluripotent stem cells (iPSCs) and isogenic controls generated using CRISPR-Cas9 mediated genome editing. The iPSCs are self-renewable and capable of being differentiated into the cell types affected by the diseases. These in vitro models based on patient-derived iPSCs provide the opportunity to model disease development, uncover novel mechanisms and test potential therapeutics. Here we review findings from iPSC-based modeling of selected neurodegenerative diseases, including Alzheimer's disease, frontotemporal dementia and spinocerebellar ataxia. Furthermore, we discuss the possibilities of generating three-dimensional (3D) models using the iPSCs-derived cells and compare their advantages and disadvantages to conventional two-dimensional (2D) models.

Exosomes as Carriers of Alzheimer's Amyloid-ß

The intracerebral level of the aggregation-prone peptide, amyloid-ß (Aß), is constantly maintained by multiple clearance mechanisms, including several degradation enzymes, and brain efflux. Disruption of the clearance machinery and the resultant Aß accumulation gives rise to neurotoxic assemblies, leading to the pathogenesis of Alzheimer's disease (AD). In addition to the classic mechanisms of Aß clearance, the protein may be processed by secreted vesicles, although this possibility has not been extensively investigated. We showed that neuronal exosomes, a subtype of extracellular nanovesicles, enwrap, or trap Aß and transport it into microglia for degradation. Here, we review Aß sequestration and elimination by exosomes, and discuss how this clearance machinery might contribute to AD pathogenesis and how it might be exploited for effective AD therapy.

Physiological and pathological roles of exosomes in the nervous system

Exosomes represent a subtype of extracellular nanovesicles that are generated from the luminal budding of limiting endosomal membranes and subsequent exocytosis. They encapsulate or associate with obsolete molecules to eliminate or to transfer their cargos in intercellular communication. The exosomes are also released and transported between neurons and glia in the nervous system, having a broad impact on nerve development, activation and regeneration. Accumulating evidence suggests that the exosomes are attributed to the pathogenesis of several neurodegenerative diseases such as prion disease, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, as well as aging, in which the exosomes lack the capacity for cellular self-repair and spread their enclosed pathological agents among neurons. In this article, we review the current proposed functions of exosomes in physiological and pathological processes in the nervous system.

Defective Transcytosis of APP and Lipoproteins in Human iPSC-Derived Neurons with Familial Alzheimer's Disease Mutations

We investigated early phenotypes caused by familial Alzheimer's disease (fAD) mutations in isogenic human iPSC-derived neurons. Analysis of neurons carrying fAD PS1 or APP mutations introduced using genome editing technology at the endogenous loci revealed that fAD mutant neurons had previously unreported defects in the recycling state of endocytosis and soma-to-axon transcytosis of APP and lipoproteins. The endocytosis reduction could be rescued through treatment with a β-secretase inhibitor. Our data suggest that accumulation of β-CTFs of APP, but not Aβ, slow vesicle formation from an endocytic recycling compartment marked by the transcytotic GTPase Rab11. We confirm previous results that endocytosis is affected in AD and extend these to uncover a neuron-specific defect. Decreased lipoprotein endocytosis and transcytosis to the axon suggest that a neuron-specific impairment in endocytic axonal delivery of lipoproteins and other key materials might compromise synaptic maintenance in fAD.

Self-Organizing 3D Human Neural Tissue Derived from Induced Pluripotent Stem Cells Recapitulate Alzheimer's Disease Phenotypes

The dismal success rate of clinical trials for Alzheimer's disease (AD) motivates us to develop model systems of AD pathology that have higher predictive validity. The advent of induced pluripotent stem cells (iPSCs) allows us to model pathology and study disease mechanisms directly in human neural cells from healthy individual as well as AD patients. However, two-dimensional culture systems do not recapitulate the complexity of neural tissue, and phenotypes such as extracellular protein aggregation are difficult to observe. We report brain organoids that use pluripotent stem cells derived from AD patients and recapitulate AD-like pathologies such as amyloid aggregation, hyperphosphorylated tau protein, and endosome abnormalities. These pathologies are observed in an age-dependent manner in organoids derived from multiple familial AD (fAD) patients harboring amyloid precursor protein (APP) duplication or presenilin1 (PSEN1) mutation, compared to controls. The incidence of AD pathology was consistent amongst several fAD lines, which carried different mutations. Although these are complex assemblies of neural tissue, they are also highly amenable to experimental manipulation. We find that treatment of patient-derived organoids with β- and γ-secretase inhibitors significantly reduces amyloid and tau pathology. Moreover, these results show the potential of this model system to greatly increase the translatability of pre-clinical drug discovery in AD.

Modeling Alzheimer's disease with human induced pluripotent stem (iPS) cells

In the last decade, induced pluripotent stem (iPS) cells have revolutionized the utility of human in vitro models of neurological disease. The iPS-derived and differentiated cells allow researchers to study the impact of a distinct cell type in health and disease as well as performing therapeutic drug screens on a human genetic background. In particular, clinical trials for Alzheimer's disease (AD) have been failing. Two of the potential reasons are first, the species gap involved in proceeding from initial discoveries in rodent models to human studies, and second, an unsatisfying patient stratification, meaning subgrouping patients based on the disease severity due to the lack of phenotypic and genetic markers. iPS cells overcome this obstacles and will improve our understanding of disease subtypes in AD. They allow researchers conducting in depth characterization of neural cells from both familial and sporadic AD patients as well as preclinical screens on human cells. In this review, we briefly outline the status quo of iPS cell research in neurological diseases along with the general advantages and pitfalls of these models. We summarize how genome-editing techniques such as CRISPR/Cas9 will allow researchers to reduce the problem of genomic variability inherent to human studies, followed by recent iPS cell studies relevant to AD. We then focus on current techniques for the differentiation of iPS cells into neural cell types that are relevant to AD research. Finally, we discuss how the generation of three-dimensional cell culture systems will be important for understanding AD phenotypes in a complex cellular milieu, and how both two- and three-dimensional iPS cell models can provide platforms for drug discovery and translational studies into the treatment of AD.

Dissecting Alzheimer disease in Down syndrome using mouse models

Down syndrome (DS) is a common genetic condition caused by the presence of three copies of chromosome 21 (trisomy 21). This greatly increases the risk of Alzheimer disease (AD), but although virtually all people with DS have AD neuropathology by 40 years of age, not all develop dementia. To dissect the genetic contribution of trisomy 21 to DS phenotypes including those relevant to AD, a range of DS mouse models has been generated which are trisomic for chromosome segments syntenic to human chromosome 21. Here, we consider key characteristics of human AD in DS (AD-DS), and our current state of knowledge on related phenotypes in AD and DS mouse models. We go on to review important features needed in future models of AD-DS, to understand this type of dementia and so highlight pathogenic mechanisms relevant to all populations at risk of AD.

Modifications of the endosomal compartment in peripheral blood mononuclear cells and fibroblasts from Alzheimer's disease patients

Identification of blood-based biomarkers of Alzheimer's disease (AD) remains a challenge. Neuropathological studies have identified enlarged endosomes in post-mortem brains as the earliest cellular change associated to AD. Here the presence of enlarged endosomes was investigated in peripheral blood mononuclear cells from 48 biologically defined AD patients (25 with mild cognitive impairment and 23 with dementia (AD-D)), and 23 age-matched healthy controls using immunocytochemistry and confocal microscopy. The volume and number of endosomes were not significantly different between AD and controls. However, the percentage of cells containing enlarged endosomes was significantly higher in the AD-D group as compared with controls. Furthermore, endosomal volumes significantly correlated to [C(11)]PiB cortical index measured by positron emission tomography in the AD group, independently of the APOE genotype, but not to the levels of amyloid-beta, tau and phosphorylated tau measured in the cerebrospinal fluid. Importantly, we confirmed the presence of enlarged endosomes in fibroblasts from six unrelated AD-D patients as compared with five cognitively normal controls. This study is the first, to our knowledge, to report morphological alterations of the endosomal compartment in peripheral cells from AD patients correlated to amyloid load that will now be evaluated as a possible biomarker.

Hippocampal endosomal, lysosomal, and autophagic dysregulation in mild cognitive impairment: correlation with aβ and tau pathology

Endosomal-lysosomal and autophagic dysregulation occurs in the hippocampus in prodromal Alzheimer disease (AD), but its relationship with β-amyloid (Aβ) and tau pathology remains unclear. To investigate this issue, we performed immunoblot analysis of hippocampal homogenates from cases with an antemortem clinical diagnosis of no cognitive impairment, mild cognitive impairment (MCI), and AD. Western blot analysis revealed significant increases in the acid hydrolase cathepsin D and early endosome marker rabaptin5 in the MCI group compared with AD, whereas levels of phosphorylated mammalian target of rapamycin proteins (pmTOR), total mammalian target of rapamycin (mTOR), p62, traf6, and LilrB2 were comparable across clinical groups. Hippocampal Aβ1-40 and Aβ1-42 concentrations and AT8-immunopositive neurofibrillary tangle density were not significantly different across the clinical groups. Greater cathepsin D expression was associated with global cognitive score and episodic memory score but not with mini mental state examination or advanced neuropathology criteria. These results indicate that alterations in hippocampal endosomal-lysosomal proteins in MCI are independent of tau or Aβ pathology.

Reprogramming of HUVECs into induced pluripotent stem cells (HiPSCs), generation and characterization of HiPSC-derived neurons and astrocytes

Neurodegenerative diseases are characterized by chronic and progressive structural or functional loss of neurons. Limitations related to the animal models of these human diseases have impeded the development of effective drugs. This emphasizes the need to establish disease models using human-derived cells. The discovery of induced pluripotent stem cell (iPSC) technology has provided novel opportunities in disease modeling, drug development, screening, and the potential for “patient-matched” cellular therapies in neurodegenerative diseases. In this study, with the objective of establishing reliable tools to study neurodegenerative diseases, we reprogrammed human umbilical vein endothelial cells (HUVECs) into iPSCs (HiPSCs). Using a novel and direct approach, HiPSCs were differentiated into cells of central nervous system (CNS) lineage, including neuronal, astrocyte and glial cells, with high efficiency. HiPSCs expressed embryonic genes such as nanog, sox2 and Oct-3/4, and formed embryoid bodies that expressed markers of the 3 germ layers. Expression of endothelial-specific genes was not detected in HiPSCs at RNA or protein levels. HiPSC-derived neurons possess similar morphology but significantly longer neurites compared to primary human fetal neurons. These stem cell-derived neurons are susceptible to inflammatory cell-mediated neuronal injury. HiPSC-derived neurons express various amino acids that are important for normal function in the CNS. They have functional receptors for a variety of neurotransmitters such as glutamate and acetylcholine. HiPSC-derived astrocytes respond to ATP and acetylcholine by elevating cytosolic Ca²⁺ concentrations. In summary, this study presents a novel technique to generate differentiated and functional HiPSC-derived neurons and astrocytes. These cells are appropriate tools for studying the development of the nervous system, the pathophysiology of various neurodegenerative diseases and the development of potential drugs for their treatments.

Retromer in Alzheimer disease, Parkinson disease and other neurological disorders

Retromer is a protein assembly that has a central role in endosomal trafficking, and retromer dysfunction has been linked to a growing number of neurological disorders. First linked to Alzheimer disease, retromer dysfunction causes a range of pathophysiological consequences that have been shown to contribute to the core pathological features of the disease. Genetic studies have established that retromer dysfunction is also pathogenically linked to Parkinson disease, although the biological mechanisms that mediate this link are only now being elucidated. Most recently, studies have shown that retromer is a tractable target in drug discovery for these and other disorders of the nervous system.

Probing the secrets of Alzheimer's disease using human-induced pluripotent stem cell technology

Our understanding of Alzheimer's disease (AD) is still incomplete and, as a result, we lack effective therapies. Reprogramming to generate human-induced pluripotent stem cells provides a new approach to the generation of human neurons that carry the genomes of people with familial or sporadic AD. Differentiation of such stem cells to human neurons is already providing new insights into AD and molecular pathways that may provide new targets for effective therapy. These pathways include typical amyloid response pathways, as well as pathways leading from altered behavior of amyloid precursor protein to the elevated phosphorylation of tau protein. There is also a need for standardization of models so that isogenic lines differing only in the familial AD mutation can be compared.

Genetic heterogeneity in Alzheimer disease and implications for treatment strategies

Since the original publication describing the illness in 1907, the genetic understanding of Alzheimer's disease (AD) has advanced such that it is now clear that it is a genetically heterogeneous condition, the subtypes of which may not uniformly respond to a given intervention. It is therefore critical to characterize the clinical and preclinical stages of AD subtypes, including the rare autosomal dominant forms caused by known mutations in the PSEN1, APP, and PSEN2 genes that are being studied in the Dominantly Inherited Alzheimer Network study and its associated secondary prevention trial. Similar efforts are occurring in an extended Colombian family with a PSEN1 mutation, in APOE ε4 homozygotes, and in Down syndrome. Despite commonalities in the mechanisms producing the AD phenotype, there are also differences that reflect specific genetic origins. Treatment modalities should be chosen and trials designed with these differences in mind. Ideally, the varying pathological cascades involved in the different subtypes of AD should be defined so that both areas of overlap and of distinct differences can be taken into account. At the very least, clinical trials should determine the influence of known genetic factors in post hoc analyses.

Human stem cell models of neurodegeneration: a novel approach to study mechanisms of disease development

The number of patients with neurodegenerative diseases is increasing significantly worldwide. Thus, intense research is being pursued to uncover mechanisms of disease development in an effort to identify molecular targets for therapeutic intervention. Analysis of postmortem tissue from patients has yielded important histological and biochemical markers of disease progression. However, this approach is inherently limited because it is not possible to study patient neurons prior to degeneration. As such, transgenic and knockout models of neurodegenerative diseases are commonly employed. While these animal models have yielded important insights into some molecular mechanisms of disease development, they do not provide the opportunity to study mechanisms of neurodegeneration in human neurons at risk and thus, it is often difficult or even impossible to replicate human pathogenesis with this approach. The generation of patient-specific induced pluripotent stem (iPS) cells offers a unique opportunity to overcome these obstacles. By expanding and differentiating iPS cells, it is possible to generate large numbers of functional neurons in vitro, which can then be used to study the disease of the donating patient. Here, we provide an overview of human stem cell models of neurodegeneration using iPS cells from patients with Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, frontotemporal dementia, Huntington's disease, spinal muscular atrophy and other neurodegenerative diseases. In addition, we describe how further refinements of reprogramming technology resulted in the generation of patient-specific induced neurons, which have also been used to model neurodegenerative changes in vitro.

Bioengineered stem cells in neural development and neurodegeneration research

The recent discovery of a simple method for making induced pluripotent stem cells (iPSC) from human somatic cells was a major scientific advancement that opened the way for many promising new developments in the study of developmental and degenerative diseases. iPSC have already been used to model many different types of neurological diseases, including autism, schizophrenia, Alzheimer's disease and Parkinson's disease. Because of their pluripotent property, iPSC offer the possibility of modeling human development in vitro. Their differentiation seems to follow the developmental timeline and obeys environmental cues. Clinically relevant phenotypes of neurodegenerative pathologies have also been observed using iPSC derived human neuronal cultures. Options for treatment are still some way off. Although some early research in mouse models has been encouraging, major obstacles remain for neural stem cell (NSC) transplantation therapy. However, iPSC now offer the prospect of an unlimited amount of human neurons or astrocytes for drug testing. The aim of this review is to summarize the recent progress in modeling neural development and neurological diseases using iPSC and to describe their applications for aging research and personalized medicine.

Trisomy for synaptojanin1 in Down syndrome is functionally linked to the enlargement of early endosomes

Enlarged early endosomes have been observed in neurons and fibroblasts in Down syndrome (DS). These endosome abnormalities have been implicated in the early development of Alzheimer's disease (AD) pathology in these subjects. Here, we show the presence of enlarged endosomes in blood mononuclear cells and lymphoblastoid cell lines (LCLs) from individuals with DS using immunofluorescence and confocal microscopy. Genotype-phenotype correlations in LCLs carrying partial trisomies 21 revealed that triplication of a 2.56 Mb locus in 21q22.11 is associated with the endosomal abnormalities. This locus contains the gene encoding the phosphoinositide phosphatase synaptojanin 1 (SYNJ1), a key regulator of the signalling phospholipid phosphatidylinositol-4,5-biphosphate that has been shown to regulate clathrin-mediated endocytosis. We found that SYNJ1 transcripts are increased in LCLs from individuals with DS and that overexpression of SYNJ1 in a neuroblastoma cell line as well as in transgenic mice leads to enlarged endosomes. Moreover, the proportion of enlarged endosomes in fibroblasts from an individual with DS was reduced after silencing SYNJ1 expression with RNA interference. In LCLs carrying amyloid precursor protein (APP) microduplications causing autosomal dominant early-onset AD, enlarged endosomes were absent, suggesting that APP overexpression alone is not involved in the modification of early endosomes in this cell type. These findings provide new insights into the contribution of SYNJ1 overexpression to the endosomal changes observed in DS and suggest an attractive new target for rescuing endocytic dysfunction and lipid metabolism in DS and in AD.

The location and trafficking routes of the neuronal retromer and its role in amyloid precursor protein transport

The retromer complex plays an important role in intracellular transport, is highly expressed in the hippocampus, and has been implicated in the trafficking of the amyloid precursor protein (APP). Nevertheless, the trafficking routes of the neuronal retromer and the role it plays in APP transport in neuronal processes remain unknown. Here we use hippocampal neuronal cultures to address these issues. Using fluorescence microscopy, we find that Vps35, the core element of the retromer complex, is in dendrites and axons, is enriched in endosomes and trans-Golgi network, and is found in APP-positive vesicles. Next, to identify the role the neuronal retromer plays in cargo transport, we infected hippocampal neurons with a lentivirus expressing shRNA to silence Vps35. By live fluorescence imaging, Vps35 deficiency was found to reduce the frequency, but not the kinetics, of long-range APP transport within neuronal processes. Supporting the interpretation that retromer promotes long-range transport, Vps35 deficiency led to increased APP in the early endosomes, in processes but not the soma. Finally, Vps35 deficiency was associated with increased levels of Aβ, a cleaved product of APP, increased colocalization of APP with its cleaving enzyme BACE1 in processes, and caused an enlargement of early endosomes. Taken together, our studies clarify the function of the neuronal retromer, and suggest specific mechanisms for how retromer dysfunction observed in Alzheimer's disease affects APP transport and processing.

Retromers in Alzheimer's disease

Amyloid-β peptide (Aβ), the key pathogenic agent in Alzheimer's disease (AD), is released after sequential proteolytic cleavage of the transmembrane amyloid precursor protein (APP). β-Site APP-cleaving enzyme 1 (BACE1) cleaves APP in early endosomes, and the cause of increased BACE cleavage of APP in AD is not fully resolved yet. It has been proposed that perturbed intracellular trafficking of APP, which leads to prolonged residence time in early endosomes, influences Aβ production and hence the risk for AD. Retromers are a family of proteins that mediate the retrieval of transmembrane proteins from the endosomes to the trans-Golgi network. Misregulation of retromers or retromer-associated proteins influences endosomal localization of APP/BACE1. Here we review the role of retromers in the amyloidogenic processing of APP and their pathogenic role in AD.

Probing sporadic and familial Alzheimer's disease using induced pluripotent stem cells

Our understanding of Alzheimer’s disease pathogenesis is currently limited by difficulties in obtaining live neurons from patients and the inability to model the sporadic form of the disease. It may be possible to overcome these challenges by reprogramming primary cells from patients into induced pluripotent stem cells (iPSCs). Here we reprogrammed primary fibroblasts from two patients with familial Alzheimer’s disease, both caused by a duplication of the amyloid-β precursor protein gene¹ (APP; termed APP^Dp), two with sporadic Alzheimer’s disease (termed sAD1, sAD2) and two non-demented control individuals into iPSC lines. Neurons from differentiated cultures were purified with fluorescence-activated cell sorting and characterized. Purified cultures contained more than 90% neurons, clustered with fetal brain messenger RNA samples by microarray criteria, and could form functional synaptic contacts. Virtually all cells exhibited normal electrophysiological activity. Relative to controls, iPSC-derived, purified neurons from the two APP^Dp patients and patient sAD2 exhibited significantly higher levels of the pathological markers amyloid-β(1–40), phospho-tau(Thr 231) and active glycogen synthase kinase-3β (aGSK-3β). Neurons from APP^Dp and sAD2 patients also accumulated large RAB5-positive early endosomes compared to controls. Treatment of purified neurons with β-secretase inhibitors, but not γ-secretase inhibitors, caused significant reductions in phospho-Tau(Thr 231) and aGSK-3β levels. These results suggest a direct relationship between APP proteolytic processing, but not amyloid-β, in GSK-3β activation and tau phosphorylation in human neurons. Additionally, we observed that neurons with the genome of one sAD patient exhibited the phenotypes seen in familial Alzheimer’s disease samples. More generally, we demonstrate that iPSC technology can be used to observe phenotypes relevant to Alzheimer’s disease, even though it can take decades for overt disease to manifest in patients.

Alzheimer's disease genetics: lessons to improve disease modelling

In the present review, we look back at the recent history of GWAS (genome-wide association studies) in AD (Alzheimer's disease) and integrate the major findings with current knowledge of biological processes and pathways. These topics are essential for the development of animal models, which will be fundamental to our complete understanding of AD.

Upregulation of select rab GTPases in cholinergic basal forebrain neurons in mild cognitive impairment and Alzheimer's disease

Endocytic system dysfunction is one of the earliest disturbances that occur in Alzheimer's disease (AD), and may underlie the selective vulnerability of cholinergic basal forebrain (CBF) neurons during the progression of dementia. Herein we report that genes regulating early and late endosomes are selectively upregulated within CBF neurons in mild cognitive impairment (MCI) and AD. Specifically, upregulation of rab4, rab5, rab7, and rab27 was observed in CBF neurons microdissected from postmortem brains of individuals with MCI and AD compared to age-matched control subjects with no cognitive impairment (NCI). Upregulated expression of rab4, rab5, rab7, and rab27 correlated with antemortem measures of cognitive decline in individuals with MCI and AD. qPCR validated upregulation of these select rab GTPases within microdissected samples of the basal forebrain. Moreover, quantitative immunoblot analysis demonstrated upregulation of rab5 protein expression in the basal forebrain of subjects with MCI and AD. The elevation of rab4, rab5, and rab7 expression is consistent with our recent observations in CA1 pyramidal neurons in MCI and AD. These findings provide further support that endosomal pathology accelerates endocytosis and endosome recycling, which may promote aberrant endosomal signaling and neurodegeneration throughout the progression of AD.

Microarray analysis of hippocampal CA1 neurons implicates early endosomal dysfunction during Alzheimer's disease progression

BACKGROUND

Endocytic dysfunction and neurotrophin signaling deficits may underlie the selective vulnerability of hippocampal neurons during the progression of Alzheimer's disease (AD), although there is little direct in vivo and biochemical evidence to support this hypothesis.

METHODS

Microarray analysis of hippocampal CA1 pyramidal neurons acquired via laser capture microdissection was performed using postmortem brain tissue. Validation was achieved using real-time quantitative polymerase chain reaction and immunoblot analysis. Mechanistic studies were performed using human fibroblasts subjected to overexpression with viral vectors or knockdown via small interference RNA.

RESULTS

Expression levels of genes regulating early endosomes (rab5) and late endosomes (rab7) are selectively upregulated in homogeneous populations of CA1 neurons from individuals with mild cognitive impairment and AD. The levels of these genes are selectively increased as antemortem measures of cognition decline during AD progression. Hippocampal quantitative polymerase chain reaction and immunoblot analyses confirmed increased levels of these transcripts and their respective protein products. Elevation of select rab GTPases regulating endocytosis paralleled the downregulation of genes encoding the neurotrophin receptors TrkB and TrkC. Overexpression of rab5 in cells suppressed TrkB expression, whereas knockdown of TrkB expression did not alter rab5 levels, suggesting that TrkB downregulation is a consequence of endosomal dysfunction associated with elevated rab5 levels in early AD.

CONCLUSIONS

These data support the hypothesis that neuronal endosomal dysfunction is associated with preclinical AD. Increased endocytic pathway activity, driven by elevated rab GTPase expression, may result in long-term deficits in hippocampal neurotrophic signaling and represent a key pathogenic mechanism underlying AD progression.

Regulation of the actin cytoskeleton-plasma membrane interplay by phosphoinositides

The plasma membrane and the underlying cortical actin cytoskeleton undergo continuous dynamic interplay that is responsible for many essential aspects of cell physiology. Polymerization of actin filaments against cellular membranes provides the force for a number of cellular processes such as migration, morphogenesis, and endocytosis. Plasma membrane phosphoinositides (especially phosphatidylinositol bis- and trisphosphates) play a central role in regulating the organization and dynamics of the actin cytoskeleton by acting as platforms for protein recruitment, by triggering signaling cascades, and by directly regulating the activities of actin-binding proteins. Furthermore, a number of actin-associated proteins, such as BAR domain proteins, are capable of directly deforming phosphoinositide-rich membranes to induce plasma membrane protrusions or invaginations. Recent studies have also provided evidence that the actin cytoskeleton-plasma membrane interactions are misregulated in a number of pathological conditions such as cancer and during pathogen invasion. Here, we summarize the wealth of knowledge on how the cortical actin cytoskeleton is regulated by phosphoinositides during various cell biological processes. We also discuss the mechanisms by which interplay between actin dynamics and certain membrane deforming proteins regulate the morphology of the plasma membrane.

Down syndrome fibroblast model of Alzheimer-related endosome pathology: accelerated endocytosis promotes late endocytic defects

Endocytic dysfunction is an early pathological change in Alzheimer's disease (AD) and Down's syndrome (DS). Using primary fibroblasts from DS individuals, we explored the interactions among endocytic compartments that are altered in AD and assessed their functional consequences in AD pathogenesis. We found that, like neurons in both AD and DS brains, DS fibroblasts exhibit increased endocytic uptake, fusion, and recycling, and trafficking of lysosomal hydrolases to rab5-positive early endosomes. Moreover, late endosomes identified using antibodies to rab7 and lysobisphosphatidic acid increased in number and appeared as enlarged, perinuclear vacuoles, resembling those in neurons of both AD and DS brains. In control fibroblasts, similar enlargement of rab5-, rab7-, and lysobisphosphatidic acid-positive endosomes was induced when endocytosis and endosomal fusion were increased by expression of either a rab5 or an active rab5 mutant, suggesting that persistent endocytic activation results in late endocytic dysfunction. Conversely, expression of a rab5 mutant that inhibits endocytic uptake reversed early and late endosomal abnormalities in DS fibroblasts. Our results indicate that DS fibroblasts recapitulate the neuronal endocytic dysfunction of AD and DS, suggesting that increased trafficking from early endosomes can account, in part, for downstream endocytic perturbations that occur in neurons in both AD and DS brains.

Innate immunity and transcription of MGAT-III and Toll-like receptors in Alzheimer's disease patients are improved by bisdemethoxycurcumin

We have tested a hypothesis that the natural product curcuminoids, which has epidemiologic and experimental rationale for use in AD, may improve the innate immune system and increase amyloid-beta (Abeta) clearance from the brain of patients with sporadic Alzheimer's disease (AD). Macrophages of a majority of AD patients do not transport Abeta into endosomes and lysosomes, and AD monocytes do not efficiently clear Abeta from the sections of AD brain, although they phagocytize bacteria. In contrast, macrophages of normal subjects transport Abeta to endosomes and lysosomes, and monocytes of these subjects clear Abeta in AD brain sections. Upon Abeta stimulation, mononuclear cells of normal subjects up-regulate the transcription of beta-1,4-mannosyl-glycoprotein 4-beta-N-acetylglucosaminyltransferase (MGAT3) (P < 0.001) and other genes, including Toll like receptors (TLRs), whereas mononuclear cells of AD patients generally down-regulate these genes. Defective phagocytosis of Abeta may be related to down-regulation of MGAT3, as suggested by inhibition of phagocytosis by using MGAT3 siRNA and correlation analysis. Transcription of TLR3, bditTLR4, TLR5, bditTLR7, TLR8, TLR9, and TLR10 upon Abeta stimulation is severely depressed in mononuclear cells of AD patients in comparison to those of control subjects. In mononuclear cells of some AD patients, the curcuminoid compound bisdemethoxycurcumin may enhance defective phagocytosis of Abeta, the transcription of MGAT3 and TLRs, and the translation of TLR2-4. Thus, bisdemethoxycurcumin may correct immune defects of AD patients and provide a previously uncharacterized approach to AD immunotherapy.

Neuroglia in the inferior olivary nucleus during normal aging and Alzheimer's disease

It is likely that neuronal loss occurs in certain brain regions in Alzheimer's Disease (AD) without any neurofibrillary pathology. In the human principle inferior olivary nucleus (PO), we have shown that neuronal loss is about 34% (Lasn et al. Journal of Alzheimer Disease, 2001; 3: 159–168), but the fate of the neuroglial cells is unknown. Since the unique network of neurons and neuroglial cells and their cohabitation are essential for normal functioning of CNS, we designed a study to estimate the total number of oligodendrocytes and astrocytes in normally aged and AD brains. The study is based on 10 control and 11 AD post-mortem human brains. An unbiased stereological fractionator method was used. We found significant oligodendroglial cell loss (46%) in AD as compared to control brains, while the total number of astrocytes showed a tendency to decrease. It is likely that the ratio of oligodendroglial cells to neurons remains unchanged even in degenerative states, indicating that oligodendroglial cells parallel neuronal loss. Astroglial cells did not increase in total number, but the ratio to neurons was significantly increased due to the neuronal loss. Using a novel unbiased quantitative method, we were able to describe significant oligodendroglial loss in the PO but the pathogenic mechanism behind remains unknown.

Targeting the role of the endosome in the pathophysiology of Alzheimer's disease: a strategy for treatment

Membrane-bound endosomal vesicles play an integral role in multiple cellular events, including protein processing and turnover, and often critically regulate the cell-surface availability of receptors and other plasma membrane proteins in many different cell types. Neurons are no exception, being dependent on endosomal function for housekeeping and synaptic events. Growing evidence suggests a link between neuronal endosomal function and Alzheimer's disease (AD) pathophysiology. Endosomal abnormalities invariably occur within neurons in AD brains, and endocytic compartments are one likely site for the production of the pathogenic beta-amyloid peptide (Abeta), which accumulates within the brain during the disease and is generated by proteolytic processing of the amyloid precursor protein (APP). The enzymes and events involved in APP processing are appealing targets for therapeutic agents aimed at slowing or reversing the pathogenesis of AD. The neuronal endosome may well prove to be the intracellular site of action for inhibitors of beta-amyloidogenic APP processing. We present here the view that knowledge of the endosomal system in the disease can guide drug discovery of AD therapeutic agents.