Increased exosome secretion in neurons aging in vitro by NPC1-mediated endosomal cholesterol buildup

The multiple facets of Rab proteins modulating the cellular distribution of cholesterol from the late endosomal compartment

Cholesterol is an essential lipid that ensures the functional integrity of mammalian cells. Most cells acquire cholesterol via endocytosis of low-density lipoproteins (LDL). Upon reaching late endosomes/lysosomes (LE/Lys), incoming ligands, including LDL-derived cholesterol, are distributed to other organelles. Niemann-Pick Type C1/2 (NPC1/2) proteins, members of the steroidogenic acute regulatory-related lipid transfer domain (StARD) and oxysterol-binding protein (OSBP) families facilitate the cellular distribution of cholesterol. NPC disease, a rare neurodegenerative disorder characterized by LE/Lys-cholesterol accumulation due to loss-of-function NPC1/2 mutations, underscores the physiological importance of LE/Lys-cholesterol distribution. Several Rab-GTPase family members, which play fundamental roles in directional membrane and lipid transport, including Rab7, 8 and 9, are critical for the delivery of cholesterol from LE/Lys to other organelles along vesicular and non-vesicular pathways. The insights gained from these regulatory circuits provide a foundation for the development of therapeutic strategies that could effectively address the cellular pathogenesis triggered by NPC1 deficiency and other lysosomal storage disorders.

Increased levels of extracellular matrix proteins associated with extracellular vesicles from brains of aged mice

Extracellular vesicles (EVs) are secreted by all major cell types of the brain, providing a mode of intercellular communication and a pathway for disposal of cellular debris. EVs help maintain healthy brain function, but may also contribute to diseases affecting the brain. EVs might contribute to aging of the brain, as aging-related processes such as inflammation and cellular senescence may alter EV cargo, promoting further inflammation and senescence. However, the effects of aging on brain EVs and the function of EVs in the aging brain remain poorly understood. To address this question, we measured the levels and protein cargo of EVs isolated from the brains of 4-, 12-, and 22-month-old C57BL/6J mice. We detected no changes in EV levels, but observed age-dependent changes in EV proteins. EV fractions from aged (22 month old) brains contained higher levels of extracellular matrix proteins than EV fractions from young (4 month old) brains, with intermediate levels in 12-month-old brains. Specifically, EV fractions from aged mice contained elevated levels of hyaluronan and proteoglycan link proteins 1 and 2 and several chondroitin sulfate proteoglycans (CSPGs). Analysis of extracellular matrix in several brain regions of aged mice revealed increased immunolabeling for the CSPG aggrecan, but reduced labeling with Wisteria floribunda agglutinin, which binds to chondroitin sulfate side chains of CSPGs. These data are consistent with prior studies showing changes to the composition of extracellular matrix in aged brains, and indicate a novel association of EVs with changes in the extracellular matrix of the aging brain.

Enhanced mGluR5 intracellular activity causes psychiatric alterations in Niemann Pick type C disease

Niemann-Pick disease Type C (NPC) is caused by mutations in the cholesterol transport protein NPC1 leading to the endolysosomal accumulation of the lipid and to psychiatric alterations. Using an NPC mouse model (Npc1nmf164) we show aberrant mGluR5 lysosomal accumulation and reduction at plasma membrane in NPC1 deficient neurons. This phenotype was induced in wild-type (wt) neurons by genetic and pharmacological NPC1 silencing. Extraction of cholesterol normalized mGluR5 distribution in NPC1-deficient neurons. Intracellular accumulation of mGluR5 was functionally active leading to enhanced mGluR-dependent long-term depression (mGluR-LTD) in Npc1nmf164 hippocampal slices. mGluR-LTD was lower or higher in Npc1nmf164 slices compared with wt when stimulated with non-membrane-permeable or membrane-permeable mGluR5 agonists, respectively. Oral treatment with the mGluR5 antagonist 2-chloro-4-((2,5-dimethyl-1-(4-(trifluoromethoxy)phenyl)-1H-imidazol-4-yl)ethynyl)pyridine (CTEP) reduced mGluR-LTD and ameliorated psychiatric anomalies in the Npc1nmf164 mice. Increased neuronal mGluR5 levels were found in an NPC patient. These results implicate mGluR5 alterations in NPC psychiatric condition and provide a new therapeutic strategy that might help patients suffering from this devastating disease.

Lysosomal cholesterol accumulation in aged astrocytes impairs cholesterol delivery to neurons and can be rescued by cannabinoids

Cholesterol is crucial for the proper functioning of eukaryotic cells, especially neurons, which rely on cholesterol to maintain their complex structure and facilitate synaptic transmission. However, brain cells are isolated from peripheral cholesterol by the blood-brain barrier and mature neurons primarily uptake the cholesterol synthesized by astrocytes for proper function. This study aimed to investigate the effect of aging on cholesterol trafficking in astrocytes and its delivery to neurons. We found that aged astrocytes accumulated high levels of cholesterol in the lysosomal compartment, and this cholesterol buildup can be attributed to the simultaneous occurrence of two events: decreased levels of the ABCA1 transporter, which impairs ApoE-cholesterol export from astrocytes, and reduced expression of NPC1, which hinders cholesterol release from lysosomes. We show that these two events are accompanied by increased microR-33 in aged astrocytes, which targets ABCA1 and NPC1. In addition, we demonstrate that the microR-33 increase is triggered by oxidative stress, one of the hallmarks of aging. By coculture experiments, we show that cholesterol accumulation in astrocytes impairs the cholesterol delivery from astrocytes to neurons. Remarkably, we found that this altered transport of cholesterol could be alleviated through treatment with endocannabinoids as well as cannabidiol or CBD. Finally, according to data demonstrating that aged astrocytes develop an A1 phenotype, we found that cholesterol buildup is also observed in reactive C3+ astrocytes. Given that reduced neuronal cholesterol affects synaptic plasticity, the ability of cannabinoids to restore cholesterol transport from aged astrocytes to neurons holds significant implications in aging and inflammation.

The complexity of extracellular vesicles: Bridging the gap between cellular communication and neuropathology

Brain-derived extracellular vesicles (EVs) serve a prominent role in maintaining homeostasis and contributing to pathology in health and disease. This review establishes a crucial link between physiological processes leading to EV biogenesis and their impacts on disease. EVs are involved in the clearance and transport of proteins and nucleic acids, responding to changes in cellular processes associated with neurodegeneration, including autophagic disruption, organellar dysfunction, aging, and other cell stresses. In neurodegenerative disorders (e.g., Alzheimer's disease, Parkinson's disease, etc.), EVs contribute to the spread of pathological proteins like amyloid β, tau, ɑ-synuclein, prions, and TDP-43, exacerbating neurodegeneration and accelerating disease progression. Despite evidence for both neuropathological and neuroprotective effects of EVs, the mechanistic switch between their physiological and pathological functions remains elusive, warranting further research into their involvement in neurodegenerative disease. Moreover, owing to their innate ability to traverse the blood-brain barrier and their ubiquitous nature, EVs emerge as promising candidates for novel diagnostic and therapeutic strategies. The review uniquely positions itself at the intersection of EV cell biology, neurophysiology, and neuropathology, offering insights into the diverse biological roles of EVs in health and disease.

The Rubicon-WIPI axis regulates exosome biogenesis during ageing

Cells release intraluminal vesicles in multivesicular bodies as exosomes to communicate with other cells. Although recent studies suggest an intimate link between exosome biogenesis and autophagy, the detailed mechanism is not fully understood. Here we employed comprehensive RNA interference screening for autophagy-related factors and discovered that Rubicon, a negative regulator of autophagy, is essential for exosome release. Rubicon recruits WIPI2d to endosomes to promote exosome biogenesis. Interactome analysis of WIPI2d identified the ESCRT components that are required for intraluminal vesicle formation. Notably, we found that Rubicon is required for an age-dependent increase of exosome release in mice. In addition, small RNA sequencing of serum exosomes revealed that Rubicon determines the fate of exosomal microRNAs associated with cellular senescence and longevity pathways. Taken together, our current results suggest that the Rubicon-WIPI axis functions as a key regulator of exosome biogenesis and is responsible for age-dependent changes in exosome quantity and quality.

Biogenesis and secretion of mitovesicles, small extracellular vesicles of mitochondrial origin at the crossroads between brain health and disease

In the brain, mitochondrial components are released into the extracellular space via several mechanisms, including a recently identified type of extracellular vesicles called mitovesicles. While vesiculation of neuronal mitochondria yields various intracellular types of vesicles, with either a single or a double membrane, mitovesicles secreted into the extracellular space are a unique subtype of these mitochondria-derived vesicles, with a double membrane and a specific set of mitochondrial DNA, RNA, proteins, and lipids. Based on the most relevant literature describing mitochondrial vesiculation and mitochondrial exocytosis, we propose a model for their secretion when the amphisome, a hybrid endosome-autophagosome organelle, fuses with the plasma membrane, releasing mitovesicles and exosomes into the extracellular space. In aging and neurodegenerative disorders, mitochondrial dysfunction, in association with endolysosomal abnormalities, alter mitovesicle number and content, with downstream effect on brain health.

Role of Toll-like receptors in exosome biogenesis and angiogenesis capacity

Adaptive inflammation consists of multiple cellular changes and molecular reactions to protect host cells against several pathological conditions. Along with the activation of varied immune cells, the production and secretion of cytokines arrays can regulate the progression of inflammatory response in a paracrine manner. Among different molecular cascades, Toll-like receptors (TLRs) are activated in response to several pathological conditions and damage signals. It has been indicated that extracellular vesicles, especially exosomes (Exos) are key bioshuttles with specific cargoes and are involved in cell-to-cell communication. The role of Exos in the initiation, progression, and cession of inflammation has been previously addressed in terms of cytokine transmission. Whether and how the activation of TLRs can alter the Exo biogenesis and angiogenesis potential in immune cells and endothelial cells (ECs) remains to be elucidated. Here, the cross-talk between the TLRs, Exo biogenesis, and angiogenesis has been highlighted.

The Aging Lacrimal Gland of Female C57BL/6J Mice Exhibits Multinucleate Macrophage Infiltration Associated With Lipid Dysregulation

Purpose

Loss of function of the lacrimal gland (LG), which produces the aqueous tear film, is implicated in age-related dry eye. To better understand this deterioration, we evaluated changes in lipid metabolism and inflammation in LGs from an aging model.

Methods

LG sections from female C57BL/6J mice of different ages (young, 2-3 months; intermediate, 10-14 months; old,  ≥24 months) were stained with Oil Red-O or Toluidine blue to detect lipids. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis and western blotting of LG lysates determined differences in the expression of genes and proteins related to lipid metabolism. A photobleaching protocol to quench age-related autofluorescence was used in LG sections to evaluate changes in immunofluorescence associated with NPC1, NPC2, CTSL, and macrophages (F4/80, CD11b) with age using confocal fluorescence microscopy.

Results

Old LGs showed increased lipids prominent in basal aggregates in acinar cells and in extra-acinar sites. LG gene expression of Npc1, Npc2, Lipa, and Mcoln2, encoding proteins involved in lipid metabolism, was increased with age. NPC1 was also significantly increased in old LGs by western blotting. In photobleached LG sections, confocal fluorescence microscopy imaging of NPC1, NPC2, and CTSL immunofluorescence showed age-associated enrichment in macrophages labeled to detect F4/80. Although mononuclear macrophages were detectable in LG at all ages, this novel multinucleate macrophage population containing NPC1, NPC2, and CTSL and enriched in F4/80 and some CD11b was increased with age at extra-acinar sites.

Conclusions

Lipid-metabolizing proteins enriched in F4/80-positive multinucleated macrophages are increased in old LGs adjacent to sites of lipid deposition in acini.

Chemo-small extracellular vesicles released in cisplatin-resistance ovarian cancer cells are regulated by the lysosomal function

Chemoresistance is a common problem in ovarian cancer (OvCa) treatment, where resistant cells, in response to chemotherapy, secrete small extracellular vesicles (sEVs), known as chemo-sEVs, that transfer resistance to recipient cells. sEVs are formed as intraluminal vesicles (ILVs) within multivesicular endosomes (MVEs), whose trafficking is regulated by Ras-associated binding (RAB) GTPases that mediate sEVs secretion or lysosomal degradation. A decrease in lysosomal function can promote sEVs secretion, but the relationship between MVEs trafficking pathways and sEVs secretion in OvCa chemoresistance is unclear. Here, we show that A2780cis cisplatin (CCDP) resistant OvCa cells had an increased number of MVEs and ILVs structures, higher levels of Endosomal Sorting Complex Required for Transport (ESCRTs) machinery components, and RAB27A compared to A2780 CDDP-sensitive OvCa cells. CDDP promoted the secretion of chemo-sEVs in A2780cis cells, enriched in DNA damage response proteins. A2780cis cells exhibited poor lysosomal function with reduced levels of RAB7, essential in MVEs-Lysosomal trafficking. The silencing of RAB27A in A2780cis cells prevents the Chemo-EVs secretion, reduces its chemoresistance and restores lysosomal function and levels of RAB7, switching them into an A2780-like cellular phenotype. Enhancing lysosomal function with rapamycin reduced chemo-sEVs secretion. Our results suggest that adjusting the balance between secretory MVEs and lysosomal MVEs trafficking could be a promising strategy for overcoming CDDP chemoresistance in OvCa.

Extracellular vesicles from senescent mesenchymal stromal cells are defective and cannot prevent osteoarthritis

Age is the most important risk factor in degenerative diseases such as osteoarthritis (OA), which is associated with the accumulation of senescent cells in the joints. Here, we aimed to assess the impact of senescence on the therapeutic properties of extracellular vesicles (EVs) from human fat mesenchymal stromal cells (ASCs) in OA. We generated a model of DNA damage-induced senescence in ASCs using etoposide and characterized EVs isolated from their conditioned medium (CM). Senescent ASCs (S-ASCs) produced 3-fold more EVs (S-EVs) with a slightly bigger size and that contain 2-fold less total RNA. Coculture experiments showed that S-ASCs were as efficient as healthy ASCs (H-ASCs) in improving the phenotype of OA chondrocytes cultured in resting conditions but were defective when chondrocytes were proliferating. S-EVs were also impaired in their capacity to polarize synovial macrophages towards an anti-inflammatory phenotype. A differential protein cargo mainly related to inflammation and senescence was detected in S-EVs and H-EVs. Using the collagenase-induced OA model, we found that contrary to H-EVs, S-EVs could not protect mice from cartilage damage and joint calcifications, and were less efficient in protecting subchondral bone degradation. In addition, S-EVs induced a pro-catabolic and pro-inflammatory gene signature in the joints of mice shortly after injection, while H-EVs decreased hypertrophic, catabolic and inflammatory pathways. In conclusion, S-EVs are functionally impaired and cannot protect mice from developing OA.

Cholesterol redistribution triggered by CYP46A1 gene therapy improves major hallmarks of Niemann-Pick type C disease but is not sufficient to halt neurodegeneration

Cholesterol 24-hydroxylase (CYP46A1) is an exclusively neuronal cytochrome P450 enzyme responsible for converting cholesterol into 24S-hydroxycholesterol, which serves as the primary pathway for eliminating cholesterol in the brain. We and others have shown that increased activity of CYP46A1 leads to reduced levels of cholesterol and has a positive effect on cognition. Therefore, we hypothesized that CYP46A1 could be a potential therapeutic target in Niemann-Pick type C (NPC) disease, a rare and fatal neurodegenerative disorder, characterized by cholesterol accumulation in endolysosomal compartments. Herein, we show that CYP46A1 ectopic expression, in cellular models of NPC and in Npc1tm(I1061T) mice by adeno-associated virus-mediated gene therapy improved NPC disease phenotype. Amelioration in functional, biochemical, molecular and neuropathological hallmarks of NPC disease were characterized. In vivo, CYP46A1 expression partially prevented weight loss and hepatomegaly, corrected the expression levels of genes involved in cholesterol homeostasis, and promoted a redistribution of brain cholesterol accumulated in late endosomes/lysosomes. Moreover, concomitant with the amelioration of cholesterol metabolism dysregulation, CYP46A1 attenuated microgliosis and lysosomal dysfunction in mouse cerebellum, favoring a pro-resolving phenotype. In vivo CYP46A1 ectopic expression improves important features of NPC disease and may represent a valid therapeutic approach to be used concomitantly with other drugs. However, promoting cholesterol redistribution does not appear to be enough to prevent Purkinje neuronal death in the cerebellum. This indicates that cholesterol buildup in neurons might not be the main cause of neurodegeneration in this human lipidosis.

Kinetic modelling of sterol transport between plasma membrane and endo-lysosomes based on quantitative fluorescence and X-ray imaging data

Niemann Pick type C1 and C2 (NPC1 and NPC2) are two sterol-binding proteins which, together, orchestrate cholesterol transport through late endosomes and lysosomes (LE/LYSs). NPC2 can facilitate sterol exchange between model membranes severalfold, but how this is connected to its function in cells is poorly understood. Using fluorescent analogs of cholesterol and quantitative fluorescence microscopy, we have recently measured the transport kinetics of sterol between plasma membrane (PM), recycling endosomes (REs) and LE/LYSs in control and NPC2 deficient fibroblasts. Here, we use kinetic modeling of this data to determine rate constants for sterol transport between intracellular compartments. Our model predicts that sterol is trapped in intraluminal vesicles (ILVs) of LE/LYSs in the absence of NPC2, causing delayed sterol export from LE/LYSs in NPC2 deficient fibroblasts. Using soft X-ray tomography, we confirm, that LE/LYSs of NPC2 deficient cells but not of control cells contain enlarged, carbon-rich intraluminal vesicular structures, supporting our model prediction of lipid accumulation in ILVs. By including sterol export via exocytosis of ILVs as exosomes and by release of vesicles-ectosomes-from the PM, we can reconcile measured sterol efflux kinetics and show that both pathways can be reciprocally regulated by the intraluminal sterol transfer activity of NPC2 inside LE/LYSs. Our results thereby connect the in vitro function of NPC2 as sterol transfer protein between membranes with its in vivo function.

Therapeutic extracellular vesicle production is substantially increased by inhibition of cellular cholesterol biosynthesis

Extracellular vesicles (EVs) are a new therapeutic modality with the promise to treat many diseases through their ability to deliver diverse molecular cargo. As with other emerging modalities transitioning into the industrialization phase, all aspects of the manufacturing process are rich with opportunities to enhance the ability to deliver these medicines to patients. With the goal of improving cell culture EV productivity, we have utilized high throughput siRNA screens to identify the underlying genetic pathways that regulate EV productivity to inform rational host cell line engineering and media development approaches. The screens identified multiple metabolic pathways of potential interest; one of which was validated and shown to be a ready implementable, cost-effective strategy to increase EV titers. We show that both EV volumetric and specific productivity from HEK293 and CHO-S were increased in a dose and cell line-dependent manner up to ninefold when cholesterol synthesis was inhibited by the inclusion of statins in the cell culture media. In addition, we show in response to statin treatment, elevation of EV markers in mesenchymal stem cell (MSC) cell culture media suggesting this approach can also be applicable to MSC EVs. Furthermore, we show that the EVs produced from statin-treated HEK293 cultures are effectively loaded by both endogenous and exogenous loading methods and have equivalent in vitro or in vivo potency relative to EVs from untreated cultures.

Mitochondrial cargo export in exosomes: Possible pathways and implication in disease biology

Exosome biogenesis occurs parallel to multiple endocytic traffic routes. These coexisting routes drive cargo loading in exosomes via overlapping of exosome biogenesis with endosomal pathways. One such pathway is autophagy which captures damaged intracellular organelles or their components in an autophagosome vesicle and route them for lysosomal degradation. However, in case of a noncanonical fusion event between autophagosome and maturing multivesicular body (MVB)-a site for exosome biogenesis, the autophagic cargo is putatively loaded in exosomes and subsequent released out of the cell via formation of an "amphisome" like structure. Similarly, during "mitophagy" or mitochondrial (mt) autophagy, amphisome formation routes mitophagy cargo to exosomes. These mt-cargo enriched exosomes or mt-enREXO are often positive for LC3 protein-an autophagic flux marker, and potent regulators of paracrine signaling with both homeostatic and pathological roles. Here, I review this emerging concept and discuss how intracellular autophagic routes helps in generation of mt-enREXO and utility of these vesicles in paracrine cellular signaling and diagnostic areas.

How statin drugs affect exosomes?

Statins reduce serum cholesterol and isoprenoids by the inhibition of cholesterol synthesis in the mevalonate pathway. Exosomes are extracellular vesicles (30-200 nm) released by all cells that regulate cell-to-cell communication in health and disease by transferring functional proteins, metabolites and nucleic acids to recipient cells. There are many reports that show an effect of statins on exosomes, from their production and release to their content and performance. In this review, we have summarized existing data on the impact of statins on the biosynthesis, secretion, content, uptake and function of exosomes.

The Role of the miR-17-92 Cluster in Autophagy and Atherosclerosis Supports Its Link to Lysosomal Storage Diseases

Establishing the role of non-coding RNA (ncRNA), especially microRNAs (miRNAs), in the regulation of cell function constitutes a current research challenge. Two to six miRNAs can act in clusters; particularly, the miR-17-92 family, composed of miR-17, miR-18a, miR-19a, miR-20a, miR-19b-1, and miR-92a is well-characterized. This cluster functions during embryonic development in cell differentiation, growth, development, and morphogenesis and is an established oncogenic cluster. However, its role in the regulation of cellular metabolism, mainly in lipid metabolism and autophagy, has received less attention. Here, we argue that the miR-17-92 cluster is highly relevant for these two processes, and thus, could be involved in the study of pathologies derived from lysosomal deficiencies. Lysosomes are related to both processes, as they control cholesterol flux and regulate autophagy. Accordingly, we compiled, analyzed, and discussed current evidence that highlights the cluster's fundamental role in regulating cellular energetic metabolism (mainly lipid and cholesterol flux) and atherosclerosis, as well as its critical participation in autophagy regulation. Because these processes are closely related to lysosomes, we also provide experimental data from the literature to support our proposal that the miR-17-92 cluster could be involved in the pathogenesis and effects of lysosomal storage diseases (LSD).

Endolysosomal cholesterol export: More than just NPC1

NPC1 plays a central role in cholesterol egress from endolysosomes, a critical step for maintaining intracellular cholesterol homeostasis. Despite recent advances in the field, the full repertoire of molecules and pathways involved in this process remains unknown. Emerging evidence suggests the existence of NPC1-independent, alternative routes. These may involve vesicular and non-vesicular mechanisms, as well as release of extracellular vesicles. Understanding the underlying molecular mechanisms that bypass NPC1 function could have important implications for the development of therapies for lysosomal storage disorders. Here we discuss how cholesterol may be exported from lysosomes in which NPC1 function is impaired.

Sex Differentially Alters Secretion of Brain Extracellular Vesicles During Aging: A Potential Mechanism for Maintaining Brain Homeostasis

Extracellular vesicles (EVs) in the brain play a role in neuronal homeostasis by removing intracellular material and regulating cell-to-cell communication. Given that sex and aging differentially modulate brain networks, we investigated sex-dependent differences in EV levels and content in the brain during aging. EVs were isolated from the brains of 3, 6, 12, 18, and 24 month-old female and male C57BL/6 J mice, and the levels of different EV species determined. While the number of plasma membrane-derived microvesicles and a subset of late endosomes-derived exosomes increased with age in the brain of female mice, no significant changes were seen in males. Mitochondria-derived mitovesicles in the brain increased during aging in both sexes, a change that may reflect aging-dependent alterations in mitochondrial function. These findings reveal enhanced turnover during aging in female brains, suggesting a mechanism for advantageous successful female brain aging and sex-depending different susceptibility to age-related neurodegenerative diseases.

Extracellular Vesicles Derived from Young Neural Cultures Attenuate Astrocytic Reactivity In Vitro

Extracellular vesicles (EVs) play an important role in intercellular communication and are involved in both physiological and pathological processes. In the central nervous system (CNS), EVs secreted from different brain cell types exert a sundry of functions, from modulation of astrocytic proliferation and microglial activation to neuronal protection and regeneration. However, the effect of aging on the biological functions of neural EVs is poorly understood. In this work, we studied the biological effects of small EVs (sEVs) isolated from neural cells maintained for 14 or 21 days in vitro (DIV). We found that EVs isolated from 14 DIV cultures reduced the extracellular levels of lactate dehydrogenase (LDH), the expression levels of the astrocytic protein GFAP, and the complexity of astrocyte architecture suggesting a role in lowering the reactivity of astrocytes, while EVs produced by 21 DIV cells did not show any of the above effects. These results in an in vitro model pave the way to evaluate whether similar results occur in vivo and through what mechanisms.