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

Unraveling the Multifaceted Roles of Extracellular Vesicles: Insights into Biology, Pharmacology, and Pharmaceutical Applications for Drug Delivery

Extracellular vesicles (EVs) are nanoparticles released from various cell types that have emerged as powerful new therapeutic option for a variety of diseases. EVs are involved in the transmission of biological signals between cells and in the regulation of a variety of biological processes, highlighting them as potential novel targets/platforms for therapeutics intervention and/or delivery. Therefore, it is necessary to investigate new aspects of EVs' biogenesis, biodistribution, metabolism, and excretion as well as safety/compatibility of both unmodified and engineered EVs upon administration in different pharmaceutical dosage forms and delivery systems. In this review, we summarize the current knowledge of essential physiological and pathological roles of EVs in different organs and organ systems. We provide an overview regarding application of EVs as therapeutic targets, therapeutics, and drug delivery platforms. We also explore various approaches implemented over the years to improve the dosage of specific EV products for different administration routes.

Cerebrospinal fluid solute transport associated with sensorimotor brain activity in rodents

Cerebrospinal fluid (CSF) is crucial for maintaining neuronal homeostasis, providing nutrition, and removing metabolic waste from the brain. However, the relationship between neuronal activity and CSF solute transport remains poorly understood. To investigate the effect of regional neuronal activity on CSF solute transport, Sprague-Dawley rats (all male, n = 30) under anesthesia received an intracisternal injection of a fluorescent tracer (Texas Red ovalbumin) and were subjected to unilateral electrical stimulation of a forelimb. Two groups (n = 10 each) underwent two different types of stimulation protocols for 90 min, one including intermittent 7.5-s resting periods and the other without rest. The control group was not stimulated. Compared to the control, the stimulation without resting periods led to increased transport across most of the cortical areas, including the ventricles. The group that received intermittent stimulation showed an elevated level of solute uptake in limited areas, i.e., near/within the ventricles and on the ventral brain surface. Interhemispheric differences in CSF solute transport were also found in the cortical regions that overlap with the forelimb sensorimotor area. These findings suggest that neuronal activity may trigger local and brain-wide increases in CSF solute transport, contributing to waste clearance.

Botulinum neurotoxin A modulates the axonal release of pathological tau in hippocampal neurons

Pathological tau aggregates propagate across functionally connected neuronal networks in human neurodegenerative pathologies, such as Alzheimer's disease. However, the mechanism underlying this process is poorly understood. Several studies have showed that tau release is dependent on neuronal activity and that pathological tau is found in the extracellular space in free form, as well as in the lumen of extracellular vesicles. We recently showed that metabotropic glutamate receptor activity and SNAP25 integrity modulate the release of pathological tau from human and mouse synaptosomes. Here, we have leveraged botulinum neurotoxins (BoNTs), which impair neurotransmitter release by cleaving specific synaptic SNARE proteins, to dissect molecular mechanisms related to tau release at synapses. In particular, we have tested the effect of botulinum neurotoxin A (BoNT/A) on the synaptic release of tau in primary mouse neurons. Hippocampal neurons were grown in microfluidic chambers and transduced with lentiviruses expressing human tau (hTau). We found that neuronal stimulation significantly increases the release of mutant hTau, whereas wild-type hTau is unaffected. Importantly, BoNT/A blocks mutant hTau release, indicating that this process is controlled by SNAP25, a component of the SNARE complex, in intact neurons. These results suggest that BoNTs are potent tools to study the spreading of pathological proteins in neurodegenerative diseases and could play a central role in identifying novel molecular targets for the development of therapeutic interventions to treat tauopathies.

TGF-β1 signalling in Alzheimer's pathology and cytoskeletal reorganization: a specialized Tau perspective

Microtubule-associated protein, Tau has been implicated in Alzheimer's disease for its detachment from microtubules and formation of insoluble intracellular aggregates within the neurons. Recent findings have suggested the expulsion of Tau seeds in the extracellular domain and their prion-like propagation between neurons. Transforming Growth Factor-β1 (TGF-β1) is a ubiquitously occurring cytokine reported to carry out immunomodulation and neuroprotection in the brain. TGF-β-mediated regulation occurs at the level of neuronal survival and differentiation, glial activation (astrocyte and microglia), amyloid production-distribution-clearance and neurofibrillary tangle formation, all of which contributes to Alzheimer's pathophysiology. Its role in the reorganization of cytoskeletal architecture and remodelling of extracellular matrix to facilitate cellular migration has been well-documented. Microglia are the resident immune sentinels of the brain responsible for surveying the local microenvironment, migrating towards the beacon of pertinent damage and phagocytosing the cellular debris or patho-protein deposits at the site of insult. Channelizing microglia to target extracellular Tau could be a good strategy to combat the prion-like transmission and seeding problem in Alzheimer's disease. The current review focuses on reaffirming the role of TGF-β1 signalling in Alzheimer's pathology and cytoskeletal reorganization and considers utilizing the approach of TGF-β-triggered microglia-mediated targeting of extracellular patho-protein, Tau, as a possible potential strategy to combat Alzheimer's disease.

The Machinery of Exosomes: Biogenesis, Release, and Uptake

Exosomes are a subtype of membrane-contained vesicles 40-200 nm in diameter that are secreted by cells into their surroundings. By transporting proteins, lipids, mRNA, miRNA, lncRNA, and DNA, exosomes are able to perform such vital functions as maintaining cellular homeostasis, removing cellular debris, and facilitating intercellular and interorgan communication. Exosomes travel in all body fluids and deliver their molecular messages in autocrine, paracrine as well as endocrine manners. In recent years, there has been an increased interest in studying exosomes as diagnostic markers and therapeutic targets, since in many disease conditions this machinery becomes dysregulated or hijacked by pathological processes. Additionally, delivery of exosomes and exosomal miRNA has already been shown to improve systemic metabolism and inhibit progression of cancer development in mice. However, the subcellular machinery of exosomes, including their biogenesis, release and uptake, remains largely unknown. This review will bring molecular details of these processes up to date with the goal of expanding the knowledge basis for designing impactful exosome experiments in the future.

Heparan Sulfate Proteoglycans in Tauopathy

Tauopathies are a class of neurodegenerative diseases, including Alzheimer's disease, and are characterized by intraneuronal tau inclusion in the brain and the patient's cognitive decline with obscure pathogenesis. Heparan sulfate proteoglycans, a major type of extracellular matrix, have been believed to involve in tauopathies. The heparan sulfate proteoglycans co-deposit with tau in Alzheimer's patient brain, directly bind to tau and modulate tau secretion, internalization, and aggregation. This review summarizes the current understanding of the functions and the modulated molecular pathways of heparan sulfate proteoglycans in tauopathies, as well as the implication of dysregulated heparan sulfate proteoglycan expression in tau pathology and the potential of targeting heparan sulfate proteoglycan-tau interaction as a novel therapeutic option.

Dysfunctional microglia and tau pathology in Alzheimer’s disease

Extensive human studies and animal models show that chronic immune system stimulation involving microglia, inflammasome, complement activation, synthesis of cytokines, and reactive oxygen species exacerbates neurodegeneration in Alzheimer’s disease (AD) and other tauopathies. Abnormalities in tau, Aβ, and microglial activation are frequently observed in dementia patients and indicate that these elements may work in concert to cause cognitive impairment. Contradicting reports from postmortem studies demonstrating the presence of Aβ aggregates in the brains of cognitively healthy individuals, as well as other investigations, show that tau aggregation is more strongly associated with synapse loss, neurodegeneration, and cognitive decline than amyloid pathology. Tau association with microtubules’ surface promotes their growth and maintains their assembly, dynamicity, and stability. In contrast, the reduced affinity of hyperphosphorylated and mislocalized tau to microtubules leads to axonal deficits and neurofibrillary tangles (NFTs). Loss of microglial neuroprotective and phagocytic functions, as indicated by the faulty clearance of amyloid plaques, as well as correlations between microglial activation and tau tangle spread, all demonstrate the critical involvement of malfunctioning microglia in driving tau propagation. This review discusses the recent reports on the contribution of microglial cells to the development and progression of tau pathology. The detailed study of pathogenic mechanisms involved in interactions between neuroinflammation and tau spread is critical in identifying the targets for efficacious treatment strategies in AD.

Unconventional secretion of tau by VAMP8 impacts its intra- and extracellular cleavage

In Alzheimer’s disease, Tau, a microtubule-associated protein, becomes hyperphosphorylated, detaches from microtubules, and accumulates in the somato-dendritic compartment where it forms insoluble aggregates. Tau also accumulates in the CSF of patients indicating that it is released by neurons. Consistent with this, several laboratories including ours have shown that Tau is secreted by neurons through unconventional secretory pathways. Recently, we reported that VAMP8, an R-SNARE found on late endosomes, increased Tau secretion and that secreted Tau was cleaved at the C-terminal. In the present study, we examined whether the increase of Tau secretion by VAMP8 affected its intra- and extracellular cleavage. Upon VAMP8 overexpression, an increase of Tau cleaved by caspase-3 in the cell lysate and medium was observed. This was correlated to an increase of active caspase-3 in the cell lysate and medium. Using a Tau mutant not cleavable by caspase-3, we demonstrated that Tau cleavage by caspase-3 was not necessary for its secretion upon VAMP8 overexpression. By adding recombinant Tau to the culture medium, we demonstrated that extracellular Tau cleavage by caspase-3 could occur because of the release of active caspase-3, which was the highest when VAMP8 was overexpressed. When cleavage of Tau by caspase-3 was prevented by using a non-cleavable mutant, secreted Tau was still cleaved at the C-terminal, the asparagine N410 contributing to it. Lastly, we demonstrated that N-terminal of Tau regulated the secretion pattern of a Tau fragment containing the microtubule-binding domain and the C-terminal of Tau upon VAMP8 overexpression. Collectively, the above observations indicate that VAMP8 overexpression affects the intra- and extracellular cleavage pattern of Tau.

Changes in rat spatial learning and memory as well as serum exosome proteins after simultaneous exposure to 1.5 GHz and 4.3 GHz microwaves

This study aimed to elucidate the effects and biological targets sensitive to simultaneous 1.5 and 4.3 GHz microwave exposure in rats. A total of 120 male Wistar rats were divided randomly into four groups: the sham (S group), 1.5 GHz microwave exposure (L group), 4.3 GHz microwave exposure (C group) and simultaneous 1.5 and 4.3 GHz microwave exposure (LC group) groups. Spatial learning and memory, cortical electrical activity, and hippocampal ultrastructure were assessed by the Morris Water Maze, electroencephalography, and transmission electron microscopy, respectively. Additionally, serum exosomes were isolated by ultracentrifugation and assessed by Western blotting, nanoparticle tracking and transmission electron microscopy. The serum exosome protein content was assessed by label-free quantitative proteomics. Impaired spatial learning and memory decreased cortical excitability, and damage to the hippocampal ultrastructure were observed in groups exposed to microwaves, especially the L and LC groups. A total of 54, 145 and 296 exosomal proteins were differentially expressed between the S group and the L, C and LC groups, respectively. These differentially expressed proteins were involved in the synaptic vesicle cycle and SNARE interactions during vesicular transport. Additionally, VAMP8, Syn7 and VMAT are potential serum markers of simultaneous microwave exposure. Thus, exposure to 1.5 and 4.3 GHz microwaves induced impairments in spatial learning and memory, and simultaneous microwave exposure had the most severe effects.

Role of SNAREs in Unconventional Secretion—Focus on the VAMP7-Dependent Secretion

Intracellular membrane protein trafficking is crucial for both normal cellular physiology and cell-cell communication. The conventional secretory route follows transport from the Endoplasmic reticulum (ER) to the plasma membrane via the Golgi apparatus. Alternative modes of secretion which can bypass the need for passage through the Golgi apparatus have been collectively termed as Unconventional protein secretion (UPS). UPS can comprise of cargo without a signal peptide or proteins which escape the Golgi in spite of entering the ER. UPS has been classified further depending on the mode of transport. Type I and Type II unconventional secretion are non-vesicular and non-SNARE protein dependent whereas Type III and Type IV dependent on vesicles and on SNARE proteins. In this review, we focus on the Type III UPS which involves the import of cytoplasmic proteins in membrane carriers of autophagosomal/endosomal origin and release in the extracellular space following SNARE-dependent intracellular membrane fusion. We discuss the role of vesicular SNAREs with a strong focus on VAMP7, a vesicular SNARE involved in exosome, lysosome and autophagy mediated secretion. We further extend our discussion to the role of unconventional secretion in health and disease with emphasis on cancer and neurodegeneration.

Concurrent suppression of Aβ aggregation and NLRP3 inflammasome activation for treating Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative illness accompanied by severe memory loss, cognitive disorders and impaired behavioral ability. Amyloid β-peptide (Aβ) aggregation and nucleotide-binding oligomerization domain (NOD)-like receptor protein 3 (NLRP3) inflammasome play crucial roles in the pathogenesis of AD. Aβ plaques not only induce oxidative stress and impair neurons, but also activate the NLRP3 inflammasome, which releases inflammatory cytokine IL-1β to trigger neuroinflammation. A bifunctional molecule, 2-[2-(benzo[d]thiazol-2-yl)phenylamino]benzoic acid (BPBA), with both Aβ-targeting and inflammasome-inhibiting capabilities was designed and synthesized. BPBA inhibited self- and Cu²⁺- or Zn²⁺-induced Aβ aggregation, disaggregated the already formed Aβ aggregates, and reduced the neurotoxicity of Aβ aggregates; it also inhibited the activation of the NLRP3 inflammasome and reduced the release of IL-1β in vitro and vivo. Moreover, BPBA decreased the production of reactive oxygen species (ROS) and alleviated Aβ-induced paralysis in transgenic C. elegans with the human Aβ₄₂ gene. BPBA exerts an anti-AD effect mainly through dissolving Aβ aggregates and inhibiting NLRP3 inflammasome activation synergistically.

Bifunctional molecule BPBA inhibits Aβ aggregation and NLRP3 inflammasome activation, thereby decreasing ROS and IL-1β in vitro and vivo; it synergistically prevents Alzheimer's disease via alleviating Aβ neurotoxicity and reducing neuroinflammation.

Immune abnormalities and differential gene expression in the hippocampus and peripheral blood of patients with Alzheimer's disease

Background

Despite decades of research, no precise mechanisms of Alzheimer's disease (AD) development have been elucidated. This study aimed to investigate novel diagnostic biomarkers in both peripheral blood cells and hippocampus tissue, and the pathogenesis of memory impairment in AD.

Methods

mRNA microarray data, including hippocampus samples (GSE1297 and GSE5281) and peripheral blood mononuclear cells (PBMCs) (GSE63060 and GSE63061), associated with AD were obtained from the Gene Expression Omnibus (GEO) database. Differentially expressed genes (DEGs) between AD and normal-aging samples were screened through a comprehensive analysis of multiple gene expression spectra after gene reannotation and batch normalization. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were used to analyze hub genes and to discover potential biomarkers related to AD. Protein-protein interaction (PPI) network maps were constructed to visualize the correlation between possible genes. The CIBERSORT algorithm was built to explore the patterns of PBMC infiltration to investigate the role of inflammation in the pathogenesis of AD.

Results

The bioinformatics analysis indicated 1,261 DEGs in the hippocampal samples and 290 in PBMCs when comparing patients with AD with normal-aging individuals. We selected 28 genes co-expressed in the hippocampus and PBMCs. A functional analysis of differential genes revealed that they were primarily involved in neuronal death, immune response, and mitochondrial function. Further, immune cell infiltration patterns demonstrated that the levels of naive CD4⁺ T cells, resting natural killer cells, M0 macrophages, and activated mast cells were higher in the peripheral blood of patients with AD, while resting memory CD4⁺ T cells were significantly lower.

Conclusions

The key gene changes present in both the hippocampus and PBMCs highly suggest their utility as an AD biomarker. In addition, according to our present results, immune abnormalities may have an important role in AD pathophysiology. When patients display these peripheral blood immune abnormalities, they may be recognized as being at high risk of developing AD.

Smuggle tau through a secret(ory) pathway

Secretion of misfolded tau, a microtubule-binding protein enriched in nerve cells, is linked to the progression of tau pathology. However, the molecular mechanisms underlying tau secretion are poorly understood. Recent work by Lee et al. [Biochemical J. (2021) 478: 1471-1484] demonstrated that the transmembrane domains of syntaxin6 and syntaxin8 could be exploited for tau release, setting a stage for testing a novel hypothesis that has profound implications in tauopathies (e.g. Alzheimer's disease, FTDP-17, and CBD/PSP) and other related neurodegenerative diseases. The present commentary highlights the importance and limitations of the study, and discusses opportunities and directions for future investigations.

Tau secretion and propagation: Perspectives for potential preventive interventions in Alzheimer's disease and other tauopathies

Alzheimer's disease (AD) is characterised by the accumulation of intracytoplasmic aggregates of tau protein, which are suggested to spread in a prion-like manner between interconnected brain regions. This spreading is mediated by the secretion and uptake of tau from the extracellular space or direct cell-to-cell transmission through cellular protrusions. The prion-like tau then converts the endogenous, normal tau into pathological forms, resulting in neurodegeneration. The endoplasmic reticulum/Golgi-independent tau secretion through unconventional secretory pathways involves delivering misfolded and aggregated tau to the plasma membrane and its release into the extracellular space by non-vesicular and vesicular mechanisms. Although cytoplasmic tau was thought to be released only from degenerating cells, studies now show that cells constitutively secrete tau at low levels under physiological conditions. The mechanisms of secretion of tau under physiological and pathological conditions remain unclear. Therefore, a better understanding of these pathways is essential for developing therapeutic approaches that can target prion-like tau forms to prevent neurodegeneration progression in AD. This review focuses on unconventional secretion pathways involved in the spread of tau pathology in AD and presents these pathways as prospective areas for future AD drug discovery and development.

Role of SNAREs in Neurodegenerative Diseases

Neurodegenerative diseases are pathologies of the central and peripheral nervous systems characterized by loss of brain functions and problems in movement which occur due to the slow and progressive degeneration of cellular elements. Several neurodegenerative diseases are known such as Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis and many studies on the molecular mechanisms underlying these pathologies have been conducted. Altered functions of some key proteins and the presence of intraneuronal aggregates have been identified as responsible for the development of the diseases. Interestingly, the formation of the SNARE complex has been discovered to be fundamental for vesicle fusion, vesicle recycling and neurotransmitter release. Indeed, inhibition of the formation of the SNARE complex, defects in the SNARE-dependent exocytosis and altered regulation of SNARE-mediated vesicle fusion have been associated with neurodegeneration. In this review, the biological aspects of neurodegenerative diseases and the role of SNARE proteins in relation to the onset of these pathologies are described.