Small Neuron-Derived Extracellular Vesicles from Individuals with Down Syndrome Propagate Tau Pathology in the Wildtype Mouse Brain

Unveiling the Intricate Link Between Anaerobe Niche and Alzheimer Disease Pathogenesis

Dysbiosis within microbiomes has been increasingly implicated in many systemic illnesses, such as cardiovascular disease, metabolic syndrome, respiratory infections, and Alzheimer disease (Ad). The correlation between Ad and microbial dysbiosis has been repeatedly shown, yet the etiologic cause of microbial dysbiosis remains elusive. From a neuropathology perspective, abnormal (often age-related) changes in the brain, associated structures, and bodily lumens tend toward an accumulation of oxygen-depleted pathologic structures, which are anaerobically selective niches. These anaerobic environments may promote progressive change in the microbial community proximal to the brain and thus deserve further investigation. In this review, we identify and explore what is known about the anaerobic niche near or associated with the brain and the anaerobes that it is harbors. We identify the anaerobe stakeholders within microbiome communities and the impacts on the neurodegenerative processes associated with Ad. Chronic oral dysbiosis in anaerobic dental pockets and the composition of the gut microbiota from fecal stool are the 2 largest anaerobic niche sources of bacterial transference to the brain. At the blood-brain barrier, cerebral atherosclerotic plaques are predominated by anaerobic species intimately associated with the brain vasculature. Focal cerebritis/brain abscess and corpora amylacea may also establish chronic anaerobic niches in direct proximity to brain parenchyma. In exploring the anaerobic niche proximal to the brain, we identify research opportunities to explore potential sources of microbial dysbiosis associated with Ad.

Amyloid-beta and tau protein beyond Alzheimer's disease

ABSTRACT

The aggregation of amyloid-beta peptide and tau protein dysregulation are implicated to play key roles in Alzheimer's disease pathogenesis and are considered the main pathological hallmarks of this devastating disease. Physiologically, these two proteins are produced and expressed within the normal human body. However, under pathological conditions, abnormal expression, post-translational modifications, conformational changes, and truncation can make these proteins prone to aggregation, triggering specific disease-related cascades. Recent studies have indicated associations between aberrant behavior of amyloid-beta and tau proteins and various neurological diseases, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis, as well as retinal neurodegenerative diseases like Glaucoma and age-related macular degeneration. Additionally, these proteins have been linked to cardiovascular disease, cancer, traumatic brain injury, and diabetes, which are all leading causes of morbidity and mortality. In this comprehensive review, we provide an overview of the connections between amyloid-beta and tau proteins and a spectrum of disorders.

Exosomes in Vascular/Neurological Disorders and the Road Ahead

Neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), stroke, and aneurysms, are characterized by the abnormal accumulation and aggregation of disease-causing proteins in the brain and spinal cord. Recent research suggests that proteins linked to these conditions can be secreted and transferred among cells using exosomes. The transmission of abnormal protein buildup and the gradual degeneration in the brains of impacted individuals might be supported by these exosomes. Furthermore, it has been reported that neuroprotective functions can also be attributed to exosomes in neurodegenerative diseases. The potential neuroprotective functions may play a role in preventing the formation of aggregates and abnormal accumulation of proteins associated with the disease. The present review summarizes the roles of exosomes in neurodegenerative diseases as well as elucidating their therapeutic potential in AD, PD, ALS, HD, stroke, and aneurysms. By elucidating these two aspects of exosomes, valuable insights into potential therapeutic targets for treating neurodegenerative diseases may be provided.

From understanding to action: Exploring molecular connections of Down syndrome to Alzheimer's disease for targeted therapeutic approach

Down syndrome (DS) is caused by a third copy of chromosome 21. Alzheimer's disease (AD) is a neurodegenerative condition characterized by the deposition of amyloid-beta (Aβ) plaques and neurofibrillary tangles in the brain. Both disorders have elevated Aβ, tau, dysregulated immune response, and inflammation. In people with DS, Hsa21 genes like APP and DYRK1A are overexpressed, causing an accumulation of amyloid and neurofibrillary tangles, and potentially contributing to an increased risk of AD. As a result, people with DS are a key demographic for research into AD therapeutics and prevention. The molecular links between DS and AD shed insights into the underlying causes of both diseases and highlight potential therapeutic targets. Also, using biomarkers for early diagnosis and treatment monitoring is an active area of research, and genetic screening for high-risk individuals may enable earlier intervention. Finally, the fundamental mechanistic parallels between DS and AD emphasize the necessity for continued research into effective treatments and prevention measures for DS patients at risk for AD. Genetic screening with customized therapy approaches may help the DS population in current clinical studies and future biomarkers.

Down Syndrome Biobank Consortium: A perspective

Individuals with Down syndrome (DS) have a partial or complete trisomy of chromosome 21, resulting in an increased risk for early-onset Alzheimer's disease (AD)-type dementia by early midlife. Despite ongoing clinical trials to treat late-onset AD, individuals with DS are often excluded. Furthermore, timely diagnosis or management is often not available. Of the genetic causes of AD, people with DS represent the largest cohort. Currently, there is a knowledge gap regarding the underlying neurobiological mechanisms of DS-related AD (DS-AD), partly due to limited access to well-characterized brain tissue and biomaterials for research. To address this challenge, we created an international consortium of brain banks focused on collecting and disseminating brain tissue from persons with DS throughout their lifespan, named the Down Syndrome Biobank Consortium (DSBC) consisting of 11 biobanking sites located in Europe, India, and the USA. This perspective describes the DSBC harmonized protocols and tissue dissemination goals.

The Role of Tau Pathology in Alzheimer's Disease and Down Syndrome

Background: Individuals with Down syndrome (DS) exhibit an almost complete penetrance of Alzheimer's disease (AD) pathology but are underrepresented in clinical trials for AD. The Tau protein is associated with microtubule function in the neuron and is crucial for normal axonal transport. In several different neurodegenerative disorders, Tau misfolding leads to hyper-phosphorylation of Tau (p-Tau), which may seed pathology to bystander cells and spread. This review is focused on current findings regarding p-Tau and its potential to seed pathology as a "prion-like" spreader. It also considers the consequences of p-Tau pathology leading to AD, particularly in individuals with Down syndrome. Methods: Scopus (SC) and PubMed (PM) were searched in English using keywords "tau AND seeding AND brain AND down syndrome". A total of 558 SC or 529 PM potentially relevant articles were identified, of which only six SC or three PM articles mentioned Down syndrome. This review was built upon the literature and the recent findings of our group and others. Results: Misfolded p-Tau isoforms are seeding competent and may be responsible for spreading AD pathology. Conclusions: This review demonstrates recent work focused on understanding the role of neurofibrillary tangles and monomeric/oligomeric Tau in the prion-like spreading of Tau pathology in the human brain.

Extracellular vesicles from the CNS play pivotal roles in neuroprotection and neurodegeneration: lessons from in vitro experiments

Intercellular communication between diverse cell types is crucial for the maintenance of the central nervous system, and exosomes have been shown to play an important role in this process. Exosomes are small extracellular vesicles (EVs) that are released by all cell types and carry cargoes that can elicit downstream effects in recipient cells. Exosomal communication in the central nervous system has been implicated in many neurodegenerative diseases, ranging from Alzheimer's disease to major depressive disorder. Though there remain many unknowns in the field of EV biology, in vitro experiments can provide many insights into their potential roles in health and disease. In this review, we discuss the findings of many in vitro EV experiments, with a focus on the potential roles in regulating cell viability, inflammation, oxidative stress, and neurite integrity in the central nervous system.

Tau seeding activity in various regions of down syndrome brain assessed by two novel assays

Propagation of tau pathology via the seeding of naive tau aggregation underlies the progression of Alzheimer's disease (AD) and related tauopathies. Individuals with Down syndrome (DS) develop tau pathology at the fourth decade of life, but tau seeding activity in DS brain has not yet been determined. To measure tau seeding activity, we developed capture assay and seeded-tau aggregation assay with truncated tau151-391. By using brain extracts from AD and related tauopathies, we validated these two methods and found that the brain extracts from AD and related tauopathies, but not from controls and the diseases in which tau was not hyperphosphorylated, captured in vitro and seeded 3R-tau151-391 and 4R-tau151-391 to aggregate in cultured cells similarly. Captured tau151-391 levels were strongly correlated with the seeded-tau151-391 aggregation. Employing these two newly developed assays, we analyzed tau seeding activity in the temporal (TC), frontal (FC), and occipital cortex (OC); corpus callosum (CC); and cerebellar cortex (CBC) of DS and control brains. We found that the extracts of TC, FC, or OC, but not the CC or CBC of DS or the corresponding brain regions of control cases, captured tau151-391. Levels of the captured tau151-391 by brain extracts were positively correlated with their levels of phosphorylated tau. Extracts of cerebral cortex and CC, but not CBC of DS with a similar tau level, induced more tau151-391 aggregation than did the corresponding samples from the control cases. Thus, higher tau seeding activity associated with tau hyperphosphorylation was found in the TC, FC, and OC of DS compared with the corresponding control regions as well as with the CBC and CC of DS. Of note, these two assays are sensitive, specific, and repeatable at a low cost and provide a platform for measuring tau seeding activity and for drug screening that targets tau propagation.

Exosomally Targeting microRNA23a Ameliorates Microvascular Endothelial Barrier Dysfunction Following Rickettsial Infection

Spotted fever group rickettsioses caused by Rickettsia (R) are devastating human infections, which mainly target microvascular endothelial cells (ECs) and can induce lethal EC barrier dysfunction in the brain and lungs. Our previous evidence reveals that exosomes (Exos) derived from rickettsial-infected ECs, namely R-ECExos, can induce disruption of the tight junctional (TJ) protein ZO-1 and barrier dysfunction of human normal recipient brain microvascular endothelial cells (BMECs). However, the underlying mechanism remains elusive. Given that we have observed that microRNA23a (miR23a), a negative regulator of endothelial ZO-1 mRNA, is selectively sorted into R-ECExos, the aim of the present study was to characterize the potential functional role of exosomal miR23a delivered by R-ECExos in normal recipient BMECs. We demonstrated that EC-derived Exos (ECExos) have the capacity to deliver oligonucleotide RNAs to normal recipient BMECs in an RNase-abundant environment. miR23a in ECExos impairs normal recipient BMEC barrier function, directly targeting TJ protein ZO-1 mRNAs. In separate studies using a traditional in vitro model and a novel single living-cell biomechanical assay, our group demonstrated that miR23a anti-sense oligonucleotide-enriched ECExos ameliorate R-ECExo-provoked recipient BMEC dysfunction in association with stabilization of ZO-1 in a dose-dependent manner. These results suggest that Exo-based therapy could potentially prove to be a promising strategy to improve vascular barrier function during bacterial infection and concomitant inflammation.

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

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

Aging in Down Syndrome: Latest Clinical Advances and Prospects

Down syndrome (DS), or trisomy 21, is the most common genetic cause of intellectual disability [...].