Age-related distribution of neuropathologic changes in the cerebral cortex of patients with Down's syndrome. Quantitative regional analysis and comparison with Alzheimer's disease

Cryo-EM structures reveal tau filaments from Down syndrome adopt Alzheimer's disease fold

Down syndrome (DS) is a common genetic condition caused by trisomy of chromosome 21. Among their complex clinical features, including musculoskeletal, neurological, and cardiovascular disabilities, individuals with DS have an increased risk of developing progressive dementia and early-onset Alzheimer's disease (AD). This dementia is attributed to the increased gene dosage of the amyloid-β (Aβ) precursor protein gene, the formation of self-propagating Aβ and tau prion conformers, and the deposition of neurotoxic Aβ plaques and tau neurofibrillary tangles. Tau amyloid fibrils have previously been established to adopt many distinct conformations across different neurodegenerative conditions. Here, we report the characterization of brain samples from four DS cases spanning 36-63 years of age by spectral confocal imaging with conformation-specific dyes and cryo-electron microscopy (cryo-EM) to determine structures of isolated tau fibrils. High-resolution structures revealed paired helical filament (PHF) and straight filament (SF) conformations of tau that were identical to those determined from AD cases. The PHFs and SFs are made of two C-shaped protofilaments, each containing a cross-β/β-helix motif. Similar to filaments from AD cases, most filaments from the DS cases adopted the PHF form, while a minority (approximately 20%) formed SFs. Samples from the youngest individual with no documented dementia had sparse tau deposits. To isolate tau for cryo-EM from this challenging sample we used a novel affinity-grid method involving a graphene oxide surface derivatized with anti-tau antibodies. This method improved isolation and revealed that primarily tau PHFs and a minor population of chronic traumatic encephalopathy type II-like filaments were present in this youngest case. These findings expand the similarities between AD and DS to the molecular level, providing insight into their related pathologies and the potential for targeting common tau filament folds by small-molecule therapeutics and diagnostics.

Symptoms and age of prodromal Alzheimer's disease in Down syndrome: a systematic review and meta-analysis

The diagnostic criteria for adult-onset Alzheimer's disease (AD) in patients with Down syndrome (DS) have not been standardised. This study investigated the specific symptoms of AD in the prodromal stage of DS, the mean age at diagnosis at each stage of dementia, and the relationship between intellectual disability (ID) and dementia. PubMed, Web of Science, and Embase were searched for studies on DS, AD, early-stage disease, initial symptoms, and prodromal dementia registered between January 2012 and January 2022. We also performed a meta-analysis of the differences between the mean age at prodromal symptoms and AD diagnosis and the proportion of mild cognitive impairment in patients with mild and moderately abnormal ID. We selected 14 articles reporting the behavioural and psychological symptoms of dementia (BPSD) and memory- and language-related impairments as early symptoms of AD in patients with DS. The specific symptoms of BPSD were classified into five categories: irritability (agitation), apathy, abnormal behaviour, adaptive functioning, and sleep disturbance. The mean age at the diagnosis of prodromal symptoms and AD dementia was 52.7 and 56.2 years, respectively (mean difference, + 3.11 years; 95% CI 1.82-4.40) in the meta-analysis. The diagnosis of mild dementia tended to correlate with ID severity (odds ratio [OR], 1.38; 95% CI 0.87-2.18). The features of behaviour-variant frontotemporal dementia may be clinically confirmed in diagnosing early symptoms of DS-associated AD (DSAD). Moreover, age-appropriate cognitive assessment is important. Further studies are required to evaluate DSAD using a combination of biomarkers and ID-related data.

Cryo-EM structures of amyloid-β and tau filaments in Down syndrome

Adult individuals with Down syndrome (DS) develop Alzheimer disease (AD). Whether there is a difference between AD in DS and AD regarding the structure of amyloid-β (Aβ) and tau filaments is unknown. Here we report the structure of Aβ and tau filaments from two DS brains. We found two Aβ40 filaments (types IIIa and IIIb) that differ from those previously reported in sporadic AD and two types of Aβ42 filaments (I and II) identical to those found in sporadic and familial AD. Tau filaments (paired helical filaments and straight filaments) were identical to those in AD, supporting the notion of a common mechanism through which amyloids trigger aggregation of tau. This knowledge is important for understanding AD in DS and assessing whether adults with DS could be included in AD clinical trials.

Cryo-EM Structures Reveal Tau Filaments from Down Syndrome Adopt Alzheimer's Disease Fold

Down syndrome (DS) is a common genetic condition caused by trisomy of chromosome 21. Among the complex clinical features including musculoskeletal, neurological and cardiovascular disabilities, individuals with DS have an increased risk of developing progressive dementia and early onset Alzheimer's Disease (AD). This is attributed to the increased gene dosage of amyloid-β (Aβ) precursor protein gene, the formation of self-propagating Aβ and tau prion conformers, and the deposition of neurotoxic Aβ plaques and tau neurofibrillary tangles. Tau amyloid fibrils have previously been established to adopt many distinct conformations across different neurodegenerative conditions. Here we report the characterization of brain samples from four DS cases spanning 36 to 63 years of age by spectral confocal imaging with conformation-specific dyes and cryo-electron microscopy (cryo-EM) to determine structures of isolated tau fibrils. High-resolution structures reveal paired helical filament (PHF) and straight filament (SF) conformations of tau that are identical to those determined from AD. The PHFs and SFs are made of two C-shaped protofilaments with a cross-β/β-helix motif. Similar to filaments from AD cases, most filaments from the DS cases adopted the PHF form, while a minority (~20%) formed SFs. Samples from the youngest individual with no documented dementia had sparse tau deposits. To isolate tau for cryo-EM from this challenging sample we used a novel affinity-grid method involving a graphene-oxide surface derivatized with anti-tau antibodies. This improved isolation and revealed primarily tau PHFs and a minor population of chronic traumatic encephalopathy type II-like filaments were present in this youngest case. These findings expand the similarities between AD and DS to the molecular level, providing insight into their related pathologies and the potential for targeting common tau filament folds by small-molecule therapeutics and diagnostics.

Early chronic fasudil treatment rescues hippocampal alterations in the Ts65Dn model for down syndrome

Down syndrome (DS) is the most common genetic disorder associated with intellectual disability. To study this syndrome, several mouse models have been developed. Among the most common is the Ts65Dn model, which mimics most of the alterations observed in DS. Ts65Dn mice, as humans with DS, show defects in the structure, density, and distribution of dendritic spines in the cerebral cortex and hippocampus. Fasudil is a potent inhibitor of the RhoA kinase pathway, which is involved in the formation and stabilization of dendritic spines. Our study analysed the effect of early chronic fasudil treatment on the alterations observed in the hippocampus of the Ts65Dn model. We observed that treating Ts65Dn mice with fasudil induced an increase in neural plasticity in the hippocampus: there was an increment in the expression of PSA-NCAM and BDNF, in the dendritic branching and spine density of granule neurons, as well as in cell proliferation and neurogenesis in the subgranular zone. Finally, the treatment reduced the unbalance between excitation and inhibition present in this model. Overall, early chronic treatment with fasudil increases cell plasticity and eliminates differences with euploid animals.

Plasma NT1-tau and Aβ42 correlate with age and cognitive function in two large Down syndrome cohorts

INTRODUCTION

People with Down syndrome (DS) often develop Alzheimer's disease (AD). Here, we asked whether ultrasensitive plasma immunoassays for a tau N-terminal fragment (NT1-tau) and Aβ isoforms predict cognitive impairment.

METHODS

Plasma NT1-tau, Aβ37 , Aβ40 , and Aβ42 levels were measured in a longitudinal discovery cohort (N = 85 participants, 220 samples) and a cross-sectional validation cohort (N = 239). We developed linear models and predicted values in the validation cohort.

RESULTS

Discovery cohort linear mixed models for NT1-tau, Aβ42 , and Aβ37:42 were significant for age; there was no main effect of time. In cross-sectional models, NT1-tau increased and Aβ42 decreased with age. NT1-tau predicted cognitive and functional scores. The discovery cohort linear model for NT1-tau predicted levels in the validation cohort.

DISCUSSION

NT1-tau correlates with age and worse cognition in DS. Further validation of NT1-tau and other plasma biomarkers of AD neuropathology in DS cohorts is important for clinical utility.

Perspectives on pain in Down syndrome

Down syndrome (DS) or trisomy 21 is a genetic condition often accompanied by chronic pain caused by congenital abnormalities and/or conditions, such as osteoarthritis, recurrent infections, and leukemia. Although DS patients are more susceptible to chronic pain as compared to the general population, the pain experience in these individuals may vary, attributed to the heterogenous structural and functional differences in the central nervous system, which might result in abnormal pain sensory information transduction, transmission, modulation, and perception. We tried to elaborate on some key questions and possible explanations in this review. Further clarification of the mechanisms underlying such abnormal conditions induced by the structural and functional differences is needed to help pain management in DS patients.

Distinct Molecular Signatures of Amyloid-Beta and Tau in Alzheimer's Disease Associated with Down Syndrome

Limited comparative data exist on the molecular spectrum of amyloid-beta (Aβ) and tau deposition in individuals with Down syndrome (DS) and sporadic Alzheimer's disease (sAD). We assessed Aβ and tau deposition severity in the temporal lobe and cerebellum of ten DS and ten sAD cases. Immunohistochemistry was performed using antibodies against eight different Aβ epitopes (6F/3D, Aβ38, Aβ39, Aβ40, Aβ42, Aβ43, pyroglutamate Aβ at third glutamic acid (AβNp3E), phosphorylated- (p-)Aβ at 8th serine (AβpSer8)), and six different pathological tau epitopes (p-Ser202/Thr205, p-Thr231, p-Ser396, Alz50, MC1, GT38). Findings were evaluated semi-quantitatively and quantitatively using digital pathology. DS cases had significantly higher neocortical parenchymal deposition (Aβ38, Aβ42, and AβpSer8), and cerebellar parenchymal deposition (Aβ40, Aβ42, AβNp3E, and AβpSer8) than sAD cases. Furthermore, DS cases had a significantly larger mean plaque size (6F/3D, Aβ42, AβNp3E) in the temporal lobe, and significantly greater deposition of cerebral and cerebellar Aβ42 than sAD cases in the quantitative analysis. Western blotting corroborated these findings. Regarding tau pathology, DS cases had significantly more severe cerebral tau deposition than sAD cases, especially in the white matter (p-Ser202/Thr205, p-Thr231, Alz50, and MC1). Greater total tau deposition in the white matter (p-Ser202/Thr205, p-Thr231, and Alz50) of DS cases was confirmed by quantitative analysis. Our data suggest that the Aβ and tau molecular signatures in DS are distinct from those in sAD.

Primary and secondary olfactory centres in human ontogeny

The olfactory centres are the evolutionary oldest and most conservative area of the telencephalon. Olfactory deficiencies are involved in a large spectrum of neurologic disorders and neurodegenerative diseases. The growing interest in human olfaction has been also been driven by COVID-19-induced transitional anosmia. Nevertheless, recent data on the human olfactory centres concerning normal histology and morphogenesis are rare. Published data in the field are mainly restricted to classic studies with non-uniform nomenclature and varied definitions of certain olfactory areas. While the olfactory system in model animals (rats, mice, and more rarely non-human primates) has been extensively investigated, the developmental timetable of olfactory centres in both human prenatal and postnatal ontogeny are poorly understood and unsystemised, which complicates the process of analysing human material, including medical researches. The main purpose of this review is to provide and discuss relevant morphological data on the normal ontogeny of the human olfactory centres, with a focus on the timetable of maturation and developmental cytoarchitecture, and with special reference to the definitions and terminology of certain olfactory areas.

Untangle the mystery behind DS-associated AD - Is APP the main protagonist?

Amyloid precursor protein profusion in Trisomy 21, also called Down Syndrome (DS), is rooted in the genetic determination of Alzheimer's disease (AD). With the recent development in patient care, the life expectancy of DS patients has gradually increased, leading to the high prospect of AD development, consequently leading to the development of plaques of amyloid proteins and neurofibrillary tangles made of tau by the fourth decade of the patient leading to dementia. The altered gene expression resulted in cellular dysfunction due to impairment of autophagy, mitochondrial and lysosomal dysfunction, and copy number variation controlled by the additional genes in Trisomy 21. The cognitive impairment and mechanistic insights underlying DS-AD conditions have been reviewed in this article. Some recent findings regarding biomarkers and therapeutics of DS-AD conditions were highlighted in this review.

RNAseq analysis of olfactory neuroepithelium cytological samples in individuals with Down syndrome compared to euploid controls: a pilot study

Down syndrome is a common genetic disorder caused by partial or complete triplication of chromosome 21. This syndrome shows an overall and progressive impairment of olfactory function, detected early in adulthood. The olfactory neuronal cells are located in the nasal olfactory mucosa and represent the first sensory neurons of the olfactory pathway. Herein, we applied the olfactory swabbing procedure to allow a gentle collection of olfactory epithelial cells in seven individuals with Down syndrome and in ten euploid controls. The aim of this research was to investigate the peripheral gene expression pattern in olfactory epithelial cells through RNAseq analysis. Validated tests (Sniffin' Sticks Extended test) were used to assess olfactory function. Olfactory scores were correlated with RNAseq results and cognitive scores (Vineland II and Leiter scales). All Down syndrome individuals showed both olfactory deficit and intellectual disability. Down syndrome individuals and euploid controls exhibited clear expression differences in genes located in and outside the chromosome 21. In addition, a significant correlation was found between olfactory test scores and gene expression, while a non-significant correlation emerged between olfactory and cognitive scores. This first preliminary step gives new insights into the Down syndrome olfactory system research, starting from the olfactory neuroepithelium, the first cellular step on the olfactory way.

Shaking up the silence: consequences of HMGN1 antagonizing PRC2 in the Down syndrome brain

Intellectual disability is a well-known hallmark of Down Syndrome (DS) that results from the triplication of the critical region of human chromosome 21 (HSA21). Major studies were conducted in recent years to gain an understanding about the contribution of individual triplicated genes to DS-related brain pathology. Global transcriptomic alterations and widespread changes in the establishment of neural lineages, as well as their differentiation and functional maturity, suggest genome-wide chromatin organization alterations in trisomy. High Mobility Group Nucleosome Binding Domain 1 (HMGN1), expressed from HSA21, is a chromatin remodeling protein that facilitates chromatin decompaction and is associated with acetylated lysine 27 on histone H3 (H3K27ac), a mark correlated with active transcription. Recent studies causatively linked overexpression of HMGN1 in trisomy and the development of DS-associated B cell acute lymphoblastic leukemia (B-ALL). HMGN1 has been shown to antagonize the activity of the Polycomb Repressive Complex 2 (PRC2) and prevent the deposition of histone H3 lysine 27 trimethylation mark (H3K27me3), which is associated with transcriptional repression and gene silencing. However, the possible ramifications of the increased levels of HMGN1 through the derepression of PRC2 target genes on brain cell pathology have not gained attention. In this review, we discuss the functional significance of HMGN1 in brain development and summarize accumulating reports about the essential role of PRC2 in the development of the neural system. Mechanistic understanding of how overexpression of HMGN1 may contribute to aberrant brain cell phenotypes in DS, such as altered proliferation of neural progenitors, abnormal cortical architecture, diminished myelination, neurodegeneration, and Alzheimer's disease-related pathology in trisomy 21, will facilitate the development of DS therapeutic approaches targeting chromatin.

Cholinergic Senescence in the Ts65Dn Mouse Model for Down Syndrome

Down syndrome (DS) induces a variable phenotype including intellectual disabilities and early development of Alzheimer’s disease (AD). Moreover, individuals with DS display accelerated aging that affects diverse organs, among them the brain. The Ts65Dn mouse is the most widely used model to study DS. Progressive loss of cholinergic neurons is one of the hallmarks of AD present in DS and in the Ts65Dn model. In this study, we quantify the number of cholinergic neurons in control and Ts65Dn mice, observing a general reduction in their number with age but in particular, a greater loss in old Ts65Dn mice. Increased expression of the m1 muscarinic receptor in the hippocampus counteracts this loss. Cholinergic neurons in the Ts65Dn mice display overexpression of the early expression gene c-fos and an increase in the expression of β-galactosidase, a marker of senescence. A possible mechanism for senescence induction could be phosphorylation of the transcription factor FOXO1 and its retention in the cytoplasm, which we are able to confirm in the Ts65Dn model. In our study, using Ts65Dn mice, we observe increased cholinergic activity, which induces a process of early senescence that culminates in the loss of these neurons.

Impact of increased APP gene dose in Down syndrome and the Dp16 mouse model

INTRODUCTION

People with Down syndrome (DS) are predisposed to Alzheimer's disease (AD). The amyloid hypothesis informs studies of AD. In AD-DS, but not sporadic AD, increased APP copy number is necessary, defining the APP gene dose hypothesis. Which amyloid precursor protein (APP) products contribute needs to be determined.

METHODS

Brain levels of full-length protein (fl-hAPP), C-terminal fragments (hCTFs), and amyloid beta (Aβ) peptides were measured in DS, AD-DS, non-demented controls (ND), and sporadic AD cases. The APP gene-dose hypothesis was evaluated in the Dp16 model.

RESULTS

DS and AD-DS differed from ND and AD for all APP products. In AD-DS, Aβ42 and Aβ40 levels exceeded AD. APP products were increased in the Dp16 model; increased APP gene dose was necessary for loss of vulnerable neurons, tau pathology, and activation of astrocytes and microglia.

DISCUSSION

Increases in APP products other than Aβ distinguished AD-DS from AD. Deciphering AD-DS pathogenesis necessitates deciphering which APP products contribute and how.

Cortical atrophy and amyloid and tau deposition in Down syndrome: A longitudinal study

Introduction: The Down syndrome population has a high prevalence for dementia, often showing their first clinical symptoms in their 40s. Methods: In a longitudinal cohort, we investigate whether amyloid deposition at time point 1 (TP1) could predict cortical thickness change at time point 2 (TP2). The association between tau burden and cortical thickness was also examined at time point 3 (TP3). Results: Between TP1 and TP2 there was pronounced cortical thinning in temporo-parietal cortices and cortical thickening in the frontal cortex. Baseline amyloid burden was strongly associated to cortical thinning progression, especially in the temporo-parietal regions. At TP3, tau deposition negatively correlated with cortical atrophy in regions where tau usually accumulates at later Braak stages. Discussion: A higher amount of amyloid accumulation triggers a cascade of changes of disease-causing processes that eventually lead to dementia. As expected, we found that regions where tau usually accumulates were those also displaying high levels of cortical atrophy.

Cell models for Down syndrome-Alzheimer’s disease research

Down syndrome (DS) is the most common chromosomal abnormality and leads to intellectual disability, increased risk of cardiac defects, and an altered immune response. Individuals with DS have an extra full or partial copy of chromosome 21 (trisomy 21) and are more likely to develop early-onset Alzheimer’s disease (AD) than the general population. Changes in expression of human chromosome 21 (Hsa21)-encoded genes, such as amyloid precursor protein (APP), play an important role in the pathogenesis of AD in DS (DS-AD). However, the mechanisms of DS-AD remain poorly understood. To date, several mouse models with an extra copy of genes syntenic to Hsa21 have been developed to characterise DS-AD-related phenotypes. Nonetheless, due to genetic and physiological differences between mouse and human, mouse models cannot faithfully recapitulate all features of DS-AD. Cells differentiated from human-induced pluripotent stem cells (iPSCs), isolated from individuals with genetic diseases, can be used to model disease-related cellular and molecular pathologies, including DS. In this review, we will discuss the limitations of mouse models of DS and how these can be addressed using recent advancements in modelling DS using human iPSCs and iPSC-mouse chimeras, and potential applications of iPSCs in preclinical studies for DS-AD.

Patterns of Mixed Pathologies in Down Syndrome

Background: Down syndrome (DS) is frequently associated with Alzheimer’s disease (AD)-related neuropathological changes. There are few observations on the spectrum of mixed proteinopathies in DS patients. Objective: This study aimed to evaluate multiple disease-associated proteinopathies in a series of DS cases. Methods: We analyzed the distribution of neurodegenerative disease associated proteins in postmortem brain samples from 11 DS cases (6 females, median age 57, range 38–66 years). Sections were stained for phosphorylated tau, 3-repeat and 4-repeat tau, amyloid-β, alpha synuclein, phosphorylated TDP-43, and P62. A comprehensive anatomical mapping and staging were applied for all proteins. Results: Tau and amyloid-β pathology was prevalent in all cases and compatible with that typically seen in AD with some subtle deviations. Four of 11 cases presented with Lewy-related pathology (LRP). Two cases followed the Braak staging (stage 4 and 5) whereas 2 cases presented with an atypical distribution. Two cases showed limbic predominant age-related TDP-43 encephalopathy (LATE) (stage 1 and stage 2) neuropathologic change. Two cases exhibited aging-related tau astrogliopathy (ARTAG). Conclusion: In addition to subtle deviations from AD regarding the morphology of Aβ deposition and distribution of neuronal tau pathology, we find that the spectrum of mixed-pathologies in DS show distinctive features such as deviations from the Braak staging of LRP and that LATE neuropathologic change and ARTAG pathology can be seen in individuals younger than in sporadic AD cases. Our observations support the notion that DS has distinctive pathogenic pathways from sporadic AD.

Developmental deficits and staging of dynamics of age associated Alzheimer's disease neurodegeneration and neuronal loss in subjects with Down syndrome

The increased life expectancy of individuals with Down syndrome (DS) is associated with increased prevalence of trisomy 21-linked early-onset Alzheimer's disease (EOAD) and dementia. The aims of this study of 14 brain regions including the entorhinal cortex, hippocampus, basal ganglia, and cerebellum in 33 adults with DS 26-72 years of age were to identify the magnitude of brain region-specific developmental neuronal deficits contributing to intellectual deficits, to apply this baseline to identification of the topography and magnitude of neurodegeneration and neuronal and volume losses caused by EOAD, and to establish age-based staging of the pattern of genetically driven neuropathology in DS. Both DS subject age and stage of dementia, themselves very strongly correlated, were strong predictors of an AD-associated decrease of the number of neurons, considered a major contributor to dementia. The DS cohort was subclassified by age as pre-AD stage, with 26-41-year-old subjects with a full spectrum of developmental deficit but with very limited incipient AD pathology, and 43-49, 51-59, and 61-72-year-old groups with predominant prevalence of mild, moderately severe, and severe dementia respectively. This multiregional study revealed a 28.1% developmental neuronal deficit in DS subjects 26-41 years of age and 11.9% AD-associated neuronal loss in DS subjects 43-49 years of age; a 28.0% maximum neuronal loss at 51-59 years of age; and a 11.0% minimum neuronal loss at 61-72 years of age. A total developmental neuronal deficit of 40.8 million neurons and AD-associated neuronal loss of 41.6 million neurons reflect a comparable magnitude of developmental neuronal deficit contributing to intellectual deficits, and AD-associated neuronal loss contributing to dementia. This highly predictable pattern of pathology indicates that successful treatment of DS subjects in the fourth decade of life may prevent AD pathology and functional decline.

Emergence of distinct and heterogeneous strains of amyloid beta with advanced Alzheimer's disease pathology in Down syndrome

Amyloid beta (Aβ) is thought to play a critical role in the pathogenesis of Alzheimer's disease (AD). Prion-like Aβ polymorphs, or "strains", can have varying pathogenicity and may underlie the phenotypic heterogeneity of the disease. In order to develop effective AD therapies, it is critical to identify the strains of Aβ that might arise prior to the onset of clinical symptoms and understand how they may change with progressing disease. Down syndrome (DS), as the most common genetic cause of AD, presents promising opportunities to compare such features between early and advanced AD. In this work, we evaluate the neuropathology and Aβ strain profile in the post-mortem brain tissues of 210 DS, AD, and control individuals. We assayed the levels of various Aβ and tau species and used conformation-sensitive fluorescent probes to detect differences in Aβ strains among individuals and populations. We found that these cohorts have some common but also some distinct strains from one another, with the most heterogeneous populations of Aβ emerging in subjects with high levels of AD pathology. The emergence of distinct strains in DS at these later stages of disease suggests that the confluence of aging, pathology, and other DS-linked factors may favor conditions that generate strains that are unique from sporadic AD.

A comparative study of pre-alpha islands in the entorhinal cortex from selected primates and in lissencephaly

The entorhinal cortex (EC) is the main interface between the sensory association areas of the neocortex and the hippocampus. It is crucial for the evaluation and processing of sensory data for long-term memory consolidation, and shows damage in many brain diseases, e.g., neurodegenerative diseases, such as Alzheimer's disease and developmental disorders. The pre-alpha layer of the EC in humans (layer II) displays a remarkable distribution of neurons in islands. These cellular islands give rise to a portion of the perforant path - the major reciprocal data stream for neocortical information into the hippocampal formation. However, the functional relevance of the morphological appearance of the pre-alpha layer in cellular islands and the precise timing of their initial appearance during primate evolution are largely unknown. Here, we conducted a comparative study of the EC from 38 non-human primates and Homo sapiens and found a strong relationship between gyrification index (GI) and the presence of the pre-alpha cellular islands. The formation of cellular islands also correlated wih brain and body weight as well as neopallial volume. In the two human lissencephalic cases, the cellular islands in the pre-alpha layer were lacking. These findings emphasize the relationship between cortical folding and island formation in the entorhinal cortex from an evolutionary perspective, and suggest a role in the pathomechanism of developmental brain disorders. This article is protected by copyright. All rights reserved.

Nerve Growth Factor Compromise in Down Syndrome

The basal forebrain cholinergic system relies on trophic support by nerve growth factor (NGF) to maintain its phenotype and function. In Alzheimer's disease (AD), basal forebrain cholinergic neurons (BFCNs) undergo progressive atrophy, suggesting a deficit in NGF trophic support. Within the central nervous system, NGF maturation and degradation are tightly regulated by an activity-dependent metabolic cascade. Here, we present a brief overview of the characteristics of Alzheimer's pathology in Down syndrome (DS) with an emphasis on this NGF metabolic pathway's disruption during the evolving Alzheimer's pathology. Such NGF dysmetabolism is well-established in Alzheimer's brains with advanced pathology and has been observed in mild cognitive impairment (MCI) and non-demented individuals with elevated brain amyloid levels. As individuals with DS inexorably develop AD, we then review findings that support the existence of a similar NGF dysmetabolism in DS coinciding with atrophy of the basal forebrain cholinergic system. Lastly, we discuss the potential of NGF-related biomarkers as indicators of an evolving Alzheimer's pathology in DS.

Elevated soluble amyloid beta protofibrils in Down syndrome and Alzheimer's disease

Down syndrome (DS) is caused by trisomy of chromosome 21, which leads to a propensity to develop amyloid β (Aβ) brain pathology in early adulthood followed later by cognitive and behavioral deterioration. Characterization of the Aβ pathology is important to better understand the clinical deterioration of DS individuals and to identify interventive strategies. Brain samples from people with DS and Alzheimer's disease (AD), as well as non-demented controls (NDC), were analyzed with respect to different Aβ species. Immunohistochemical staining using antibodies towards Aβ was also performed. Elevated levels of soluble Aβ protofibrils and insoluble Aβx-40 and Aβx-42 in formic acid brain extracts, and elevated immunohistochemical staining of Aβ deposits were demonstrated with the antibody BAN2401 (lecanemab) in DS and AD compared with NDC. These data and the promising data in a large phase 2 CE clinical trial with lecanemab suggest that lecanemab may have the potential to preserve cognitive capacity in DS. Lecanemab is currently in a phase 3 CE clinical trial.

Characterization of monomeric and soluble aggregated Aβ in Down's syndrome and Alzheimer's disease brains

The major characteristics of Alzheimer's disease (AD) are amyloid plaques, consisting of aggregated beta amyloid (Aβ) peptides, together with tau pathology (tangles, neuropil treads and dystrophic neurites surrounding the plaques), in the brain. Down's syndrome (DS) individuals are at increased risk to develop AD-type pathology; most DS individuals have developed substantial pathology already at the age of 40. DS individuals have an extra copy of chromosome 21, harbouring the amyloid precursor protein gene (APP). Our aim was to investigate the Aβ peptide pattern in DS and AD brains to investigate differences in their amyloid deposition and aggregation, respectively. Cortical tissue from patients with DS (with amyloid pathology), sporadic AD and controls were homogenized and fractionated into TBS (water soluble) and formic acid (water insoluble) fractions. Immunoprecipitation (IP) was performed using a variety of antibodies targeting different Aβ species including oligomeric Aβ. Mass spectrometry was then used to evaluate the presence of Aβ species in the different patient groups. A large number of Aβ peptides were identified including Aβ1-X, 2-X, 3-X, 4-X, 5-X, 11-X, and Aβ peptides extended N terminally of the BACE1 cleavage site and ending at amino 15 in the Aβ sequence APP/Aβ(-X to 15), as well as peptides post-translationally modified by pyroglutamate formation. Most Aβ peptides had higher abundance in AD and DS compared to controls, except the APP/Aβ(-X to 15) peptides which were most abundant in DS followed by controls and AD. Furthermore, the abundancies of AβX-40 and AβX-34 were increased in DS compared with AD. Aβ1-40, Aβ1-42, and Aβ4-42 were identified as the main constitutes of protofibrils (IP'd using mAb158) and higher relative Aβ1-42 signals were obtained compared with samples IP'd with 6E10 + 4G8, indicating that the protofibrils/oligomers were enriched with peptides ending at amino acid 42. All Aβ peptides found in AD were also present in DS indicating similar pathways of Aβ peptide production, degradation and accumulation, except for APP/Aβ(-X to 15). Likewise, the Aβ peptides forming protofibrils/oligomers in both AD and DS were similar, implying the possibility that treatment with clinical benefit in sporadic AD might also be beneficial for subjects with DS.

Cerebellar Calcium-Binding Protein and Neurotrophin Receptor Defects in Down Syndrome and Alzheimer's Disease

Cerebellar hypoplasia is a major characteristic of the Down syndrome (DS) brain. However, the consequences of trisomy upon cerebellar Purkinje cells (PC) and interneurons in DS are unclear. The present study performed a quantitative and qualitative analysis of cerebellar neurons immunostained with antibodies against calbindin D-28k (Calb), parvalbumin (Parv), and calretinin (Calr), phosphorylated and non-phosphorylated intermediate neurofilaments (SMI-34 and SMI-32), and high (TrkA) and low (p75NTR) affinity nerve growth factor (NGF) receptors as well as tau and amyloid in DS (n = 12), Alzheimer's disease (AD) (n = 10), and healthy non-dementia control (HC) (n = 8) cases. Our findings revealed higher Aβ42 plaque load in DS compared to AD and HC but no differences in APP/Aβ plaque load between HC, AD, and DS. The cerebellar cortex neither displayed Aβ40 containing plaques nor pathologic phosphorylated tau in any of the cases examined. The number and optical density (OD) measurements of Calb immunoreactive (-ir) PC soma and dendrites were similar between groups, while the number of PCs positive for Parv and SMI-32 were significantly reduced in AD and DS compared to HC. By contrast, the number of SMI-34-ir PC dystrophic axonal swellings, termed torpedoes, was significantly greater in AD compared to DS. No differences in SMI-32- and Parv-ir PC OD measurements were observed between groups. Conversely, total number of Parv- (stellate/basket) and Calr (Lugaro, brush, and Golgi)-positive interneurons were significantly reduced in DS compared to AD and HC. A strong negative correlation was found between counts for Parv-ir interneurons, Calr-ir Golgi and brush cells, and Aβ42 plaque load. Number of TrkA and p75NTR positive PCs were reduced in AD compared to HC. These findings suggest that disturbances in calcium binding proteins play a critical role in cerebellar neuronal dysfunction in adults with DS.

Amyloid Precursor Protein Variant, E665D, Associated With Unique Clinical and Biomarker Phenotype

We describe a clinical, imaging and biomarker phenotype associated with an amyloid precursor gene (APP) E665D variant in a 45-year-old man with progressive cognitive and behavioral dysfunction. Brain MRI showed bilateral, confluent T2 hyperintensities predominantly in the anterior white matter. Amyloid imaging and CSF testing were consistent with amyloid deposition. 7 Tesla MRI revealed cerebral microhemorrhages suggestive of cerebral amyloid angiopathy (CAA). Contrary to previous reports, this case raises the possibility that the APP E665D genetic change may be pathogenic, particularly given the abnormal Alzheimer's disease biomarkers observed in the cerebrospinal fluid, positive amyloid imaging and imaging evidence for CAA in a relatively young patient with progressive cognitive decline.

Piriform cortex alterations in the Ts65Dn model for down syndrome

The piriform cortex is involved in olfactory information processing, that is altered in Down Syndrome. Moreover, piriform cortex has a crucial involvement in epilepsy generation and is one of the first regions affected in Alzheimer's Disease, both maladies being prevalent among Down Syndrome individuals. In this work, we studied the alterations in neuronal morphology, synaptology and structural plasticity in the piriform cortex of the Ts65Dn mouse model, which is the most used model for the study of this syndrome and mimics some of their alterations. We have observed that Ts65Dn piriform cortex displays: a reduction in dendritic arborisation, a higher density of inhibitory synapses (GAD67), a lower density of excitatory synapses (vGLUT1) and a higher density of inhibitory postsynaptic puncta (gephyrin). Under electron microscopy the excitatory presynaptic and postsynaptic elements were larger in trisomic mice than in controls. Similar results were obtained using confocal microscopy. There were less immature neurons in piriform cortex layer II in addition to a reduction in the expression of PSA-NCAM in the neuropil that subsequently can reflect impairment in structural plasticity. These data support the idea of an impaired environment with altered ratio of inhibition and excitation that involves a reduction in plasticity and dendritic atrophy, providing a possible substrate for the olfactory processing impairment observed in DS individuals.

The Alzheimer's Biomarker Consortium-Down Syndrome: Rationale and methodology

INTRODUCTION

Adults with Down syndrome (DS) are at exceptionally high risk for Alzheimer's disease (AD), with virtually all individuals developing key neuropathological features by age 40. Identifying biomarkers of AD progression in DS can provide valuable insights into pathogenesis and suggest targets for disease modifying treatments.

METHODS

We describe the development of a multi-center, longitudinal study of biomarkers of AD in DS. The protocol includes longitudinal examination of clinical, cognitive, blood and cerebrospinal fluid-based biomarkers, magnetic resonance imaging and positron emission tomography measures (at 16-month intervals), as well as genetic modifiers of AD risk and progression.

RESULTS

Approximately 400 individuals will be enrolled in the study (more than 370 to date). The methodological approach from the administrative, clinical, neuroimaging, omics, neuropathology, and statistical cores is provided.

DISCUSSION

This represents the largest U.S.-based, multi-site, biomarker initiative of AD in DS. Findings can inform other multidisciplinary networks studying AD in the general population.

Alzheimer's Disease in Down Syndrome: Progress in the Design and Conduct of Drug Prevention Trials

Individuals with Down syndrome (DS) are at high risk for developing Alzheimer's disease (AD) pathology and this has provided significant insights into our understanding of the genetic basis of AD. The present review summarizes recent clinical, neuropathologic, imaging, and fluid biomarker studies of AD in DS (DSAD), highlighting the striking similarities, as well as some notable differences, between DSAD and the more common late-onset form of AD (LOAD) in the general population, as well as the much rarer, autosomal-dominant form of AD (ADAD). There has been significant progress in our understanding of the natural history of AD biomarkers in DS and their relationship to clinically meaningful changes. Additional work is needed to clearly define the continuum of AD that has been described in the general population, such as the preclinical, prodromal, and dementia stages of AD. Multiple therapeutic approaches, including those targeting not only β-amyloid but also tau and the amyloid precursor protein itself, require consideration. Recent developments in the field are presented within the context of such efforts to conduct clinical trials to treat and potentially prevent AD in DS.

Diagnosis of prodromal and Alzheimer's disease dementia in adults with Down syndrome using neuropsychological tests

INTRODUCTION

We aimed to define prodromal Alzheimer's disease (AD) and AD dementia using normative neuropsychological data in a large population-based cohort of adults with Down syndrome (DS).

METHODS

Cross-sectional study. DS participants were classified into asymptomatic, prodromal AD and AD dementia, based on neurologist's judgment blinded to neuropsychological data (Cambridge Cognitive Examination for Older Adults with Down's syndrome [CAMCOG-DS] and modified Cued Recall Test [mCRT]). We compared the cutoffs derived from the normative data in young adults with DS to those from receiver-operating characteristic curve (ROC) analysis.

RESULTS

Diagnostic performance of the CAMCOG-DS and modified Cued Recall Test (mCRT) in subjects with mild and moderate levels of intellectual disability (ID) was high, both for diagnosing prodromal AD and AD dementia (area under the curve [AUC] 0.73-0.83 and 0.90-1, respectively). The cutoffs derived from the normative data were similar to those derived from the ROC analyses.

DISCUSSION

Diagnosing prodromal AD and AD dementia in DS with mild and moderate ID using population norms for neuropsychological tests is possible with high diagnostic accuracy.

Amnestic and non-amnestic symptoms of dementia: An international study of Alzheimer's disease in people with Down's syndrome

The presence of age-related neuropathology characteristic of Alzheimer's disease (AD) in people with Down syndrome (DS) is well-established. However, the early symptoms of dementia may be atypical and appear related to dysfunction of prefrontal circuitry.

OBJECTIVE

To characterize the initial informant reported age-related neuropsychiatric symptoms of dementia in people with DS, and their relationship to AD and frontal lobe function.

METHODS

Non-amnestic informant reported symptoms (disinhibition, apathy, and executive dysfunction) and amnestic symptoms from the CAMDEX-DS informant interview were analyzed in a cross-sectional cohort of 162 participants with DS over 30 years of age, divided into three groups: stable cognition, prodromal dementia, and AD. To investigate age-related symptoms prior to evidence of prodromal dementia we stratified the stable cognition group by age.

RESULTS

Amnestic and non-amnestic symptoms were present before evidence of informant-reported cognitive decline. In those who received the diagnosis of AD, symptoms tended to be more marked. Memory impairments were more marked in the prodromal dementia than the stable cognition group (OR = 35.07; P < .001), as was executive dysfunction (OR = 7.16; P < .001). Disinhibition was greater in the AD than in the prodromal dementia group (OR = 3.54; P = .04). Apathy was more pronounced in the AD than in the stable cognition group (OR = 34.18; P < .001).

CONCLUSION

Premorbid amnestic and non-amnestic symptoms as reported by informants increase with the progression to AD. For the formal diagnosis of AD in DS this progression of symptoms needs to be taken into account. An understanding of the unique clinical presentation of DS in AD should inform treatment options.

Brain insulin resistance triggers early onset Alzheimer disease in Down syndrome

Dysregulation of insulin signaling pathway with reduced downstream neuronal survival and plasticity mechanisms is a fundamental abnormality observed in Alzheimer's disease (AD) brain. This phenomenon, known as brain insulin resistance, is associated with poor cognitive performance and is driven by the uncoupling of insulin receptor (IR) from its direct substrate (IRS1). Considering that Down syndrome (DS) and AD neuropathology share many common features, we investigated metabolic aspects of neurodegeneration, i.e., brain insulin resistance, in DS and whether it would contribute to early onset AD in DS population. Changes of levels and activation of main brain proteins belonging to the insulin signaling pathway (i.e., IR, IRS1, PTEN, GSK3β, PKCζ, AS160, GLUT4) were evaluated. Furthermore, we analyzed whether changes of these proteins were associated with alterations of: (i) proteins regulating brain energy metabolism; (ii) APP cleavage; and (ii) regulation of synaptic plasticity mechanisms in post-mortem brain samples collected from people with DS before and after the development of AD pathology (DSAD) compared with their age-matched controls. We found that DS cases were characterized by key markers of brain insulin resistance (reduced IR and increased IRS1 inhibition) early in life. Furthermore, downstream from IRS1, an overall uncoupling among the proteins of insulin signaling was observed. Dysregulated brain insulin signaling was associated with reduced hexokinase II (HKII) levels and proteins associated with mitochondrial complexes levels as well as with reduced levels of syntaxin in DS cases. Tellingly, these alterations precede the development of AD neuropathology and clinical presentations in DS. We propose that markers of brain insulin resistance rise earlier with age in DS compared with the general population and may contribute to the cognitive impairment associated with the early development of AD in DS.

Down syndrome and Alzheimer's disease: common molecular traits beyond the amyloid precursor protein

Alzheimer’s disease (AD) is the most prevalent type of dementia. Down syndrome (DS) is the leading genetic risk factor for Early-Onset AD, prematurely presenting the classic pathological features of the brain with AD. Augmented gene dosage, including the APP gene, could partially cause this predisposition. Recent works have revealed that alterations in chromosome location due to the extra Chromosome 21, as well as epigenetic modifications, could promote changes in gene expression other than those from Chromosome 21. As a result, similar pathological features and cellular dysfunctions in DS and AD, including impaired autophagy, lysosomal activity, and mitochondrial dysfunction, could be controlled beyond APP overexpression. In this review, we highlight some recent data regarding the origin of the shared features between DS and AD and explore the mechanisms concerning cognitive deficiencies in DS associated with dementia, which could shed some light into the search for new therapeutic targets for AD treatment.

Integrating Biomarker Outcomes into Clinical Trials for Alzheimer's Disease in Down Syndrome

The NIH-funded Alzheimer's Biomarker Consortium Down Syndrome (ABC-DS) and the European Horizon 21 Consortium are collecting critical new information on the natural history of Alzheimer's Disease (AD) biomarkers in adults with Down syndrome (DS), a population genetically predisposed to developing AD. These studies are also providing key insights into which biomarkers best represent clinically meaningful outcomes that are most feasible in clinical trials. This paper considers how these data can be integrated in clinical trials for individuals with DS. The Alzheimer's Clinical Trial Consortium - Down syndrome (ACTC-DS) is a platform that brings expert researchers from both networks together to conduct clinical trials for AD in DS across international sites while building on their expertise and experience.

Aberrant Oligodendrogenesis in Down Syndrome: Shift in Gliogenesis?

Down syndrome (DS), or trisomy 21, is the most prevalent chromosomal anomaly accounting for cognitive impairment and intellectual disability (ID). Neuropathological changes of DS brains are characterized by a reduction in the number of neurons and oligodendrocytes, accompanied by hypomyelination and astrogliosis. Recent studies mainly focused on neuronal development in DS, but underestimated the role of glial cells as pathogenic players. Aberrant or impaired differentiation within the oligodendroglial lineage and altered white matter functionality are thought to contribute to central nervous system (CNS) malformations. Given that white matter, comprised of oligodendrocytes and their myelin sheaths, is vital for higher brain function, gathering knowledge about pathways and modulators challenging oligodendrogenesis and cell lineages within DS is essential. This review article discusses to what degree DS-related effects on oligodendroglial cells have been described and presents collected evidence regarding induced cell-fate switches, thereby resulting in an enhanced generation of astrocytes. Moreover, alterations in white matter formation observed in mouse and human post-mortem brains are described. Finally, the rationale for a better understanding of pathways and modulators responsible for the glial cell imbalance as a possible source for future therapeutic interventions is given based on current experience on pro-oligodendroglial treatment approaches developed for demyelinating diseases, such as multiple sclerosis.

Neuropathological correlates of amyloid PET imaging in Down syndrome

Down syndrome (DS) results in an overproduction of amyloid-β (Aβ) peptide associated with early onset of Alzheimer's disease (AD). DS cases have Aβ deposits detectable histologically as young as 12-30 years of age, primarily in the form of diffuse plaques, the type of early amyloid pathology also seen at pre-clinical (i.e., pathological aging) and prodromal stages of sporadic late onset AD. In DS subjects aged >40 years, levels of cortical Aβ deposition are similar to those observed in late onset AD and in addition to diffuse plaques involve cored plaques associated with dystrophic neurites (neuritic plaques), which are of neuropathological diagnostic significance in AD. The purpose of this review is to summarize and discuss findings from amyloid PET imaging studies of DS in reference to postmortem amyloid-based neuropathology. PET neuroimaging applied to subjects with DS has the potential to (a) track the natural progression of brain pathology, including the earliest stages of amyloid accumulation, and (b) determine whether amyloid PET biomarkers predict the onset of dementia. In addition, the question that is still incompletely understood and relevant to both applications is the ability of amyloid PET to detect Aβ deposits in their earliest form.

Exosome release and cargo in Down syndrome

Down syndrome (DS) is a multisystem disorder affecting 1 in 800 births worldwide. Advancing technology, medical treatment, and social intervention have dramatically increased life expectancy, yet there are many etiologies of this disorder that are in need of further research. The advent of the ability to capture extracellular vesicles (EVs) in blood from specific cell types allows for the investigation of novel intracellular processes. Exosomes are one type of EVs that have demonstrated great potential in uncovering new biomarkers of neurodegeneration and disease, and also that appear to be intricately involved in the transsynaptic spread of pathogenic factors underlying Alzheimer's disease and other neurological diseases. Exosomes are nanosized vesicles, generated in endosomal multivesicular bodies (MVBs) and secreted by most cells in the body. Since exosomes are important mediators of intercellular communication and genetic exchange, they have emerged as a major research focus and have revealed novel biological sequelae involved in conditions afflicting the DS population. This review summarizes current knowledge on exosome biology in individuals with DS, both early in life and in aging individuals. Collectively these studies have demonstrated that complex multicellular processes underlying DS etiologies may include abnormal formation and secretion of extracellular vesicles such as exosomes.

Maternal Choline Supplementation Alters Basal Forebrain Cholinergic Neuron Gene Expression in the Ts65Dn Mouse Model of Down Syndrome

Down syndrome (DS), trisomy 21, is marked by intellectual disability and a premature aging profile including degeneration of the basal forebrain cholinergic neuron (BFCN) projection system, similar to Alzheimer's disease (AD). Although data indicate that perinatal maternal choline supplementation (MCS) alters the structure and function of these neurons in the Ts65Dn mouse model of DS and AD (Ts), whether MCS affects the molecular profile of vulnerable BFCNs remains unknown. We investigated the genetic signature of BFCNs obtained from Ts and disomic (2N) offspring of Ts65Dn dams maintained on a MCS diet (Ts+, 2N+) or a choline normal diet (ND) from mating until weaning, then maintained on ND until 4.4-7.5 months of age. Brains were then collected and prepared for choline acetyltransferase (ChAT) immunohistochemistry and laser capture microdissection followed by RNA extraction and custom-designed microarray analysis. Findings revealed upregulation of select transcripts in classes of genes related to the cytoskeleton (Tubb4b), AD (Cav1), cell death (Bcl2), presynaptic (Syngr1), immediate early (Fosb, Arc), G protein signaling (Gabarap, Rgs10), and cholinergic neurotransmission (Chrnb3) in Ts compared to 2N mice, which were normalized with MCS. Moreover, significant downregulation was seen in select transcripts associated with the cytoskeleton (Dync1h1), intracellular signaling (Itpka, Gng3, and Mlst8), and cell death (Ccng1) in Ts compared to 2N mice that was normalized with MCS. This study provides insight into genotype-dependent differences and the effects of MCS at the molecular level within a key vulnerable cell type in DS and AD.

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.

Prion and Prion-Like Protein Strains: Deciphering the Molecular Basis of Heterogeneity in Neurodegeneration

Increasing evidence suggests that neurodegenerative disorders share a common pathogenic feature: the presence of deposits of misfolded proteins with altered physicochemical properties in the Central Nervous System. Despite a lack of infectivity, experimental data show that the replication and propagation of neurodegenerative disease-related proteins including amyloid-β (Aβ), tau, α-synuclein and the transactive response DNA-binding protein of 43 kDa (TDP-43) share a similar pathological mechanism with prions. These observations have led to the terminology of "prion-like" to distinguish between conditions with noninfectious characteristics but similarities with the prion replication and propagation process. Prions are considered to adapt their conformation to changes in the context of the environment of replication. This process is known as either prion selection or adaptation, where a distinct conformer present in the initial prion population with higher propensity to propagate in the new environment is able to prevail over the others during the replication process. In the last years, many studies have shown that prion-like proteins share not only the prion replication paradigm but also the specific ability to aggregate in different conformations, i.e., strains, with relevant clinical, diagnostic and therapeutic implications. This review focuses on the molecular basis of the strain phenomenon in prion and prion-like proteins.

Frontal cortex and striatal cellular and molecular pathobiology in individuals with Down syndrome with and without dementia

Although, by age 40, individuals with Down syndrome (DS) develop amyloid-β (Aβ) plaques and tau-containing neurofibrillary tangles (NFTs) linked to cognitive impairment in Alzheimer's disease (AD), not all people with DS develop dementia. Whether Aβ plaques and NFTs are associated with individuals with DS with (DSD +) and without dementia (DSD -) is under-investigated. Here, we applied quantitative immunocytochemistry and fluorescent procedures to characterize NFT pathology using antibodies specific for tau phosphorylation (pS422, AT8), truncation (TauC3, MN423), and conformational (Alz50, MC1) epitopes, as well as Aβ and its precursor protein (APP) to frontal cortex (FC) and striatal tissue from DSD + to DSD - cases. Expression profiling of single pS422 labeled FC layer V and VI neurons was also determined using laser capture microdissection and custom-designed microarray analysis. Analysis revealed that cortical and striatal Aβ plaque burdens were similar in DSD + and DSD - cases. In both groups, most FC plaques were neuritic, while striatal plaques were diffuse. By contrast, FC AT8-positive NFTs and neuropil thread densities were significantly greater in DSD + compared to DSD -, while striatal NFT densities were similar between groups. FC pS422-positive and TauC3 NFT densities were significantly greater than Alz50-labeled NFTs in DSD + , but not DSD - cases. Putaminal, but not caudate pS422-positive NFT density, was significantly greater than TauC3-positive NFTs. In the FC, AT8 + pS422 + Alz50, TauC3 + pS422 + Alz50, pS422 + Alz50, and TauC3 + pS422 positive NFTs were more frequent in DSD + compared to DSD- cases. Single gene-array profiling of FC pS422 positive neurons revealed downregulation of 63 of a total of 864 transcripts related to Aβ/tau biology, glutamatergic, cholinergic, and monoaminergic metabolism, intracellular signaling, cell homeostasis, and cell death in DSD + compared DSD - cases. These observations suggest that abnormal tau aggregation plays a critical role in the development of dementia in DS.

Brain Development Measured With MRI in Children With Down Syndrome Correlates With Blood Biochemical Biomarkers

BACKGROUND

Down syndrome (DS) is a neurodegenerative disease with unknown mechanisms. β-Amyloid peptide (Aβ) and tau protein (Tau) are known to play a role, while vitamin A (VA) has an effect on normal neurological function. In a case-control study, we quantitatively evaluated whole brain and hippocampal volumes of DS children and analyzed the correlation of hippocampal volumes with blood levels of Aβ, Tau and VA.

METHODS

All subjects underwent magnetic resonance imaging (MRI) of the brain. The whole brain and hippocampal volumes were quantitatively analyzed using voxel-based morphometry (VBM) and stereology respectively. The blood levels of Aβ, Tau, and VA were detected by enzyme-linked immunosorbent assay and high-performance liquid chromatography, respectively.

RESULTS

Thirty DS children and twenty healthy controls were recruited. Whole brain and hippocampal volumes were significantly smaller in individuals with DS than in healthy controls. In both groups, whole brain and hippocampal volumes increased in accordance with age. The results of correlation analysis suggested that Aβ42/Aβ40 and VA are associated with hippocampal volume in DS patients.

CONCLUSION

DS children exhibited neurodevelopmental defects, even at an early age. Aβ42/Aβ40 and VA may affect hippocampal volume in DS patients.

Dementia in Down syndrome: unique insights for Alzheimer disease research

Virtually all adults with Down syndrome (DS) show the neuropathological changes of Alzheimer disease (AD) by the age of 40 years. This association is partially due to overexpression of amyloid precursor protein, encoded by APP, as a result of the location of this gene on chromosome 21. Amyloid-β accumulates in the brain across the lifespan of people with DS, which provides a unique opportunity to understand the temporal progression of AD and the epigenetic factors that contribute to the age of dementia onset. This age dependency in the development of AD in DS can inform research into the presentation of AD in the general population, in whom a longitudinal perspective of the disease is not often available. Comparison of the risk profiles, biomarker profiles and genetic profiles of adults with DS with those of individuals with AD in the general population can help to determine common and distinct pathways as well as mechanisms underlying increased risk of dementia. This Review evaluates the similarities and differences between the pathological cascades and genetics underpinning DS and AD with the aim of providing a platform for common exploration of these disorders.

Down syndrome

Down syndrome (DS; Trisomy 21) is the most common chromosomal disorder in humans. It has numerous associated neurologic phenotypes including intellectual disability, sleep apnea, seizures, behavioral problems, and dementia. With improved access to medical care, people with DS are living longer than ever before. As more individuals with DS reach old age, the necessity for further life span research is essential and cannot be overstated. There is currently a scarcity of information on common medical conditions encountered as individuals with DS progress into adulthood and old age. Conflicting information and uncertainty about the relative risk of dementia for adults with DS is a source of distress for the DS community that creates a major obstacle to proper evaluation and treatment. In this chapter, we discuss the salient neurologic phenotypes of DS, including Alzheimer's disease (AD), and current understanding of their biologic bases and management.

Chemosensory dysfunction in neurodegenerative diseases

A number of neurodegenerative diseases are accompanied by disordered smell function. The degree of dysfunction can vary among different diseases, such that olfactory testing can aid in differentiating, for example, Alzheimer's disease (AD) from major affective disorder and Parkinson's disease (PD) from progressive supranuclear palsy. Unfortunately, altered smell function often goes unrecognized by patients and physicians alike until formal testing is undertaken. Such testing uniquely probes brain regions not commonly examined in physical examinations and can identify, in some cases, patients who are already in the "preclinical" stage of disease. Awareness of this fact is one reason why the Quality Standards Committee of the American Academy of Neurology has designated smell dysfunction as one of the key diagnostic criteria for PD. The same recommendation has been made by the Movement Disorder Society for both the diagnosis of PD and identification of prodromal PD. Similar suggestions are proposed to include olfactory dysfunction as an additional research criterion for the diagnosis of AD. Although taste impairment, i.e., altered sweet, sour, bitter, salty, and umami perception, has also been demonstrated in some disorders, taste has received much less scientific attention than smell. In this review, we assess what is known about the smell and taste disorders of a wide range of neurodegenerative diseases and describe studies seeking to understand their pathologic underpinnings.

A tau homeostasis signature is linked with the cellular and regional vulnerability of excitatory neurons to tau pathology

Excitatory neurons are preferentially impaired in early Alzheimer's disease but the pathways contributing to their relative vulnerability remain largely unknown. Here we report that pathological tau accumulation takes place predominantly in excitatory neurons compared to inhibitory neurons, not only in the entorhinal cortex, a brain region affected in early Alzheimer's disease, but also in areas affected later by the disease. By analyzing RNA transcripts from single-nucleus RNA datasets, we identified a specific tau homeostasis signature of genes differentially expressed in excitatory compared to inhibitory neurons. One of the genes, BCL2-associated athanogene 3 (BAG3), a facilitator of autophagy, was identified as a hub, or master regulator, gene. We verified that reducing BAG3 levels in primary neurons exacerbated pathological tau accumulation, whereas BAG3 overexpression attenuated it. These results define a tau homeostasis signature that underlies the cellular and regional vulnerability of excitatory neurons to tau pathology.

Tau PET Imaging for Staging of Alzheimer's Disease in Down Syndrome

Alzheimer's disease (AD) pathology and early-onset dementia develop almost universally in Down syndrome (DS). AD is defined neuropathologically by the presence of extracellular plaques of aggregated amyloid β protein and intracellular neurofibrillary tangles (NFTs) of aggregated hyperphosphorylated tau protein. The development of radiolabeled positron emission tomography (PET) ligands for amyloid plaques and tau tangles enables the longitudinal assessment of the spatial pattern of their accumulation in relation to symptomatology. Recent work indicates that amyloid pathology develops 15-20 years before neurodegeneration and symptom onset in the sporadic and autosomal dominant forms of AD, while tau pathology correlates more closely with symptomatic stages evidenced by cognitive decline and dementia. Recent work on AD biomarkers in DS illustrates similarities between DS and sporadic AD. It may soon be possible to apply recently developed staging classifications to DS to obtain a more nuanced understanding of the development AD in DS and to provide more accurate diagnosis and prognosis in the clinic.

Striking reduction in neurons and glial cells in anterior thalamic nuclei of older patients with Down syndrome

The anterior thalamic nuclei are important for spatial and episodic memory, however, surprisingly little is known about the status of these nuclei in neurological conditions that present with memory impairments, such as Down syndrome. We quantified neurons and glial cells in the anterior thalamic nuclei of four older patients with Down syndrome. There was a striking reduction in the volume of the anterior thalamic nuclei and this appeared to reflect the loss of approximately 70% of neurons. The number of glial cells was also reduced but to a lesser degree than neurons. The anterior thalamic nuclei appear to be particularly sensitive to effects of aging in Down syndrome and the pathology in this region likely contributes to the memory impairments observed. These findings reaffirm the importance of examining the status of the anterior thalamic nuclei in conditions where memory impairments have been principally assigned to pathology in the medial temporal lobe.

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Volume of anterior thalamus is markedly reduced in older patients with Down syndrome.

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Number of neurons in anterior thalamus are substantially reduced.

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Number of glial cells in anterior thalamus are substantially reduced.