Structural insights into fusion mechanisms of small extracellular vesicles with model plasma membranes

Mechanical property estimation of sarcoma-relevant extracellular vesicles using transmission electron microscopy

Analysis of single extracellular vesicles (EVs) has the potential to yield valuable label-free information on their morphological structure, biomarkers and therapeutic targets, though such analysis is hindered by the lack of reliable and quantitative measurements of the mechanical properties of these compliant nanoscale particles. The technical challenge in mechanical property measurements arises from the existing tools and methods that offer limited throughput, and the reported elastic moduli range over several orders of magnitude. Here, we report on a flow-based method complemented by transmission electron microscopy (TEM) imaging to provide a high throughput, whole EV deformation analysis for estimating the mechanical properties of liposarcoma-derived EVs as a function of their size. Our study includes extracting morphological data of EVs from a large dataset of 432 TEM images, with images containing single to multiple EVs, and implementing the thin-shell deformation theory. We estimated the elastic modulus, E = 0.16 ± 0.02 MPa (mean±SE) for small EVs (sEVs; 30-150 nm) and E = 0.17 ± 0.03 MPa (mean±SE) for large EVs (lEVs; >150 nm). To our knowledge, this is the first report on the mechanical property estimation of LPS-derived EVs and has the potential to establish a relationship between EV size and EV mechanical properties.

Small-angle X-ray and neutron scattering applied to lipid-based nanoparticles: Recent advancements across different length scales

Lipid-based nanoparticles (LNPs), ranging from nanovesicles to non-lamellar assemblies, have gained significant attention in recent years, as versatile carriers for delivering drugs, vaccines, and nutrients. Small-angle scattering methods, employing X-rays (SAXS) or neutrons (SANS), represent unique tools to unveil structure, dynamics, and interactions of such particles on different length scales, spanning from the nano to the molecular scale. This review explores the state-of-the-art on scattering methods applied to unveil the structure of lipid-based nanoparticles and their interactions with drugs and bioactive molecules, to inform their rational design and formulation for medical applications. We will focus on complementary information accessible with X-rays or neutrons, ranging from insights on the structure and colloidal processes at a nanoscale level (SAXS) to details on the lipid organization and molecular interactions of LNPs (SANS). In addition, we will review new opportunities offered by Time-resolved (TR)-SAXS and -SANS for the investigation of dynamic processes involving LNPs. These span from real-time monitoring of LNPs structural evolution in response to endogenous or external stimuli (TR-SANS), to the investigation of the kinetics of lipid diffusion and exchange upon interaction with biomolecules (TR-SANS). Finally, we will spotlight novel combinations of SAXS and SANS with complementary on-line techniques, recently enabled at Large Scale Facilities for X-rays and neutrons. This emerging technology enables synchronized multi-method investigation, offering exciting opportunities for the simultaneous characterization of the structure and chemical or mechanical properties of LNPs.

In Situ Labeling of the Aqueous Compartment of Extracellular Vesicles with Luminescent Gold Nanoclusters

Extracellular vesicles (EVs) are well-known membrane-limited particles secreted by both healthy and cancerous cells. They are considered as biomarkers for early cancer diagnosis and are involved in many pathologies and physiological pathways. They could serve as diagnostic tools in liquid biopsies, as therapeutics in regenerative medicine, or as drug delivery vehicles. Our aim is here to encapsulate luminescent nanoprobes in the aqueous compartment of human EVs extracted from reproductive fluids. The analysis and labeling of the EVs content with easily detectable luminescent nanoparticles could enable a powerful tool for early diagnosis of specific diseases and also for the design of new therapeutics. In this view, gold nanoclusters (AuNCs) appear as an attractive alternative as nontoxic fluorophore probes because of their luminescence properties, large window of fluorescence lifetimes (1 ns-1 μs), ultrasmall size (<2 nm), good biocompatibility, and specific ability as X-ray photosensitizers. Here, we investigated an attractive method that uses fusogenic liposomes to deliver gold nanoclusters into EVs. This approach guarantees the preservation of the EVs membrane without any breakage, thus maintaining compartmental integrity. Different lipid compositions of liposomes preloaded with AuNCs were selected to interact electrostatically with human EVs and compared in terms of fusion efficiency. The mixture of liposomes and EVs results in membrane mixing as demonstrated by FRET experiments and fusion revealed by flux cytometry and cryo-TEM. The resulting fused EVs exhibit typical fluorescence of the AuNCs together with an increased size in agreement with fusion. Moreover, the fusion events in mixtures of EVs and AuNCs preloaded liposomes were analyzed by using cryo-electron microscopy. Finally, the ratio of released AuNCs during the fusion between the fusogenic liposomes and the EVs was estimated to be less than 20 mol % by Au titration using ICP spectroscopy.

Biogenesis and delivery of extracellular vesicles: harnessing the power of EVs for diagnostics and therapeutics

Extracellular vesicles (EVs) are membrane-enclosed particles secreted by a variety of cell types. These vesicles encapsulate a diverse range of molecules, including proteins, nucleic acids, lipids, metabolites, and even organelles derived from their parental cells. While EVs have emerged as crucial mediators of intercellular communication, they also hold immense potential as both biomarkers and therapeutic agents for numerous diseases. A thorough understanding of EV biogenesis is crucial for the development of EV-based diagnostic developments since the composition of EVs can reflect the health and disease status of the donor cell. Moreover, when EVs are taken up by target cells, they can exert profound effects on gene expression, signaling pathways, and cellular behavior, which makes these biomolecules enticing targets for therapeutic interventions. Yet, despite decades of research, the intricate processes underlying EV biogenesis by donor cells and subsequent uptake by recipient cells remain poorly understood. In this review, we aim to summarize current insights and advancements in the biogenesis and uptake mechanisms of EVs. By shedding light on the fundamental mechanisms governing EV biogenesis and delivery, this review underscores the potential of basic mechanistic research to pave the way for developing novel diagnostic strategies and therapeutic applications.

International Society for Extracellular Vesicles Workshop. QuantitatEVs: multiscale analyses, from bulk to single extracellular vesicle

The 'QuantitatEVs: multiscale analyses, from bulk to single vesicle' workshop aimed to discuss quantitative strategies and harmonized wet and computational approaches toward the comprehensive analysis of extracellular vesicles (EVs) from bulk to single vesicle analyses with a special focus on emerging technologies. The workshop covered the key issues in the quantitative analysis of different EV-associated molecular components and EV biophysical features, which are considered the core of EV-associated biomarker discovery and validation for their clinical translation. The in-person-only workshop was held in Trento, Italy, from January 31st to February 2nd, 2023, and continued in Milan on February 3rd with "Next Generation EVs", a satellite event dedicated to early career researchers (ECR). This report summarizes the main topics and outcomes of the workshop.

Lipid bilayer fluidity and degree of order regulates small EVs adsorption on model cell membrane

Small extracellular vesicles (sEVs) are known to play an important role in the communication between distant cells and to deliver biological information throughout the body. To date, many studies have focused on the role of sEVs characteristics such as cell origin, surface composition, and molecular cargo on the resulting uptake by the recipient cell. Yet, a full understanding of the sEV fusion process with recipient cells and in particular the role of cell membrane physical properties on the uptake are still lacking. Here we explore this problem using sEVs from a cellular model of triple-negative breast cancer fusing to a range of synthetic planar lipid bilayers both with and without cholesterol, and designed to mimic the formation of 'raft'-like nanodomains in cell membranes. Using time-resolved Atomic Force Microscopy we were able to track the sEVs interaction with the different model membranes, showing the process to be strongly dependent on the local membrane fluidity. The strongest interaction and fusion is observed over the less fluid regions, with sEVs even able to disrupt ordered domains at sufficiently high cholesterol concentration. Our findings suggest the biophysical characteristics of recipient cell membranes to be crucial for sEVs uptake regulation.

Thinking small to win big? A critical review on the potential application of extracellular vesicles for biomarker discovery and new therapeutic approaches in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is an extremely deadly form of cancer, with limited progress in 5-year survival rates despite significant research efforts. The main challenges in treating PDAC include difficulties in early detection, and resistance to current therapeutic approaches due to aggressive molecular and microenvironment features. These challenges emphasize the importance of identifying clinically validated biomarkers for early detection and clinical management. Extracellular vesicles (EVs), particularly exosomes, have emerged as crucial mediators of intercellular communication by transporting molecular cargo. Recent research has unveiled their role in initiation, metastasis, and chemoresistance of PDAC. Consequently, utilizing EVs in liquid biopsies holds promise for the identification of biomarkers for early detection, prognosis, and monitoring of drug efficacy. However, numerous limitations, including challenges in isolation and characterization of homogeneous EVs populations, as well as the absence of standardized protocols, can affect the reliability of studies involving EVs as biomarkers, underscoring the necessity for a prudent approach. EVs have also garnered considerable attention as a promising drug delivery system and novel therapy for tumors. The loading of biomolecules or chemical drugs into exosomes and their subsequent delivery to target cells can effectively impede tumor progression. Nevertheless, there are obstacles that must be overcome to ensure the accuracy and efficacy of therapies relying on EVs for the treatment of tumors. In this review, we examine both recent advancements and remaining obstacles, exploring the potential of utilizing EVs in biomarker discovery as well as for the development of drug delivery vehicles.

Electrostatic interactions control the adsorption of extracellular vesicles onto supported lipid bilayers

Communication between cells located in different parts of an organism is often mediated by membrane-enveloped nanoparticles, such as extracellular vesicles (EVs). EV binding and cell uptake mechanisms depend on the heterogeneous composition of the EV membrane. From a colloidal perspective, the EV membrane interacts with other biological interfaces via both specific and non-specific interactions, where the latter include long-ranged electrostatic and van der Waals forces, and short-ranged repulsive "steric-hydration" forces. While electrostatic forces are generally exploited in most EV immobilization protocols, the roles played by various colloidal forces in controlling EV adsorption on surfaces have not yet been thoroughly addressed. In the present work, we study the adsorption of EVs onto supported lipid bilayers (SLBs) carrying different surface charge densities using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and confocal laser scanning microscopy (CLSM). We demonstrate that EV adsorption onto lipid membranes can be controlled by varying the strength of electrostatic forces and we theoretically describe the observed phenomena within the framework of nonlinear Poisson-Boltzmann theory. Our modelling results confirm the experimental observations and highlight the crucial role played by attractive electrostatics in EV adsorption onto lipid membranes. They furthermore show that simplified theories developed for model lipid systems can be successfully applied to the study of their biological analogues and provide new fundamental insights into EV-membrane interactions with potential use in developing novel EV separation and immobilization strategies.

Quantitative and functional characterisation of extracellular vesicles after passive loading with hydrophobic or cholesterol-tagged small molecules

Extracellular vesicles (EVs) are nanosized intercellular messengers that bear enormous application potential as biological drug delivery vehicles. Much progress has been made for loading or decorating EVs with proteins, peptides or RNAs using genetically engineered donor cells, but post-isolation loading with synthetic drugs and using EVs from natural sources remains challenging. In particular, quantitative and unambiguous data assessing whether and how small molecules associate with EVs versus other components in the samples are still lacking. Here we describe the systematic and quantitative characterisation of passive EV loading with small molecules based on hydrophobic interactions - either through direct adsorption of hydrophobic compounds, or by membrane anchoring of hydrophilic ligands via cholesterol tags. As revealed by single vesicle imaging, both ligand types bind to CD63 positive EVs (exosomes), however also non-specifically to other vesicles, particles, and serum proteins. The hydrophobic compounds Curcumin and Terbinafine aggregate on EVs with no apparent saturation up to 106-107 molecules per vesicle as quantified by liquid chromatography - high resolution mass spectrometry (LC-HRMS). For both compounds, high density EV loading resulted in the formation of a population of large, electron-dense vesicles as detected by quantitative cryo-transmission electron microscopy (TEM), a reduced EV cell uptake and a toxic gain of function for Curcumin-EVs. In contrast, cholesterol tagging of a hydrophilic mdm2-targeted cyclic peptide saturated at densities of ca 104-105 molecules per vesicle, with lipidomics showing addition to, rather than replacement of endogenous cholesterol. Cholesterol anchored ligands did not change the EVs' size or morphology, and such EVs retained their cell uptake activity without inducing cell toxicity. However, the cholesterol-anchored ligands were rapidly shed from the vesicles in presence of serum. Based on these data, we conclude that (1) both methods allow loading of EVs with small molecules but are prone to unspecific compound binding or redistribution to other components if present in the sample, (2) cholesterol anchoring needs substantial optimization of formulation stability for in vivo applications, whereas (3) careful titration of loading densities is warranted when relying on hydrophobic interactions of EVs with hydrophobic compounds to mitigate changes in physicochemical properties, loss of EV function and potential cell toxicity.

MSC-EVs alleviate osteoarthritis by regulating microenvironmental cells in the articular cavity and maintaining cartilage matrix homeostasis

Osteoarthritis (OA), a common cause of chronic articular cartilage degeneration, is the main cause of disability in older adults and severely affects quality of life. Multiple factors are involved in the pathogenesis of OA, resulting in imbalance in the homeostasis of the joint cavity microenvironment, which exacerbates the disease. Because of the deficiency of blood vessels and nerves in cartilage, existing therapies to promote cartilage healing are relatively ineffective. Mesenchymal stem cell (MSC)-related therapies have achieved positive outcomes for the treatment of OA, and these beneficial effects have been confirmed to be largely mediated by extracellular vesicles (EVs). MSC-derived EVs (MSC-EVs) have been demonstrated to participate in the regulation of chondrocyte function, to have anti-inflammatory and immunomodulatory effects, and to alleviate metabolic disorders of the extracellular matrix, thereby slowing the progression of OA. In addition, engineered MSC-EVs can enrich therapeutic molecules and optimize administration to enhance their therapeutic effects on OA. A thorough understanding of the endogenous properties of EVs and related engineering strategies could help researchers develop more precise control therapy for OA.

A compendium of single extracellular vesicle flow cytometry

Flow cytometry (FCM) offers a multiparametric technology capable of characterizing single extracellular vesicles (EVs). However, most flow cytometers are designed to detect cells, which are larger than EVs. Whereas cells exceed the background noise, signals originating from EVs partly overlap with the background noise, thereby making EVs more difficult to detect than cells. This technical mismatch together with complexity of EV-containing fluids causes limitations and challenges with conducting, interpreting and reproducing EV FCM experiments. To address and overcome these challenges, researchers from the International Society for Extracellular Vesicles (ISEV), International Society for Advancement of Cytometry (ISAC), and the International Society on Thrombosis and Haemostasis (ISTH) joined forces and initiated the EV FCM working group. To improve the interpretation, reporting, and reproducibility of future EV FCM data, the EV FCM working group published an ISEV position manuscript outlining a framework of minimum information that should be reported about an FCM experiment on single EVs (MIFlowCyt-EV). However, the framework contains limited background information. Therefore, the goal of this compendium is to provide the background information necessary to design and conduct reproducible EV FCM experiments. This compendium contains background information on EVs, the interaction between light and EVs, FCM hardware, experimental design and preanalytical procedures, sample preparation, assay controls, instrument data acquisition and calibration, EV characterization, and data reporting. Although this compendium focuses on EVs, many concepts and explanations could also be applied to FCM detection of other particles within the EV size range, such as bacteria, lipoprotein particles, milk fat globules, and viruses.

Dysmyelination and glycolipid interference caused by phenylalanine in phenylketonuria

Phenylketonuria (PKU) is a metabolic disorder connected to an excess of phenylalanine (Phe) in the blood and tissues, with neurological consequences. The disease's molecular bases seem to be related to the accumulation of Phe at the cell membrane surface. Radiological outcomes in the brain demonstrate decreased water diffusivity in white matter, involving axon dysmyelination of not yet understood origin. We used a biophysical approach and model membranes to extend our knowledge of Phe-membrane interaction by clarifying Phe's propensity to affect membrane structure and dynamics based on lipid composition, with emphasis on modulating cholesterol and glycolipid components to mimic raft domains and myelin sheath membranes. Phe showed affinity for the investigated membrane mimics, mainly affecting the Phe-facing membrane leaflet. The surfaces of our neuronal membrane raft mimics were strong anchoring sites for Phe, showing rigidifying effects. From a therapeutic perspective, we further investigated the role of doxycycline, known to disturb Phe packing, unveiling its action as a competitor in Phe interactions with the membrane, suggesting its potential for treatment in the early stages of PKU. Our results suggest how Phe accumulation in extracellular fluids can impede normal growth of myelin sheaths by interfering with membrane slipping and by remodulating free water and myelin-associated water contents.

Unconventional Protein Secretion Dependent on Two Extracellular Vesicles: Exosomes and Ectosomes

In addition to conventional protein secretion, dependent on the specific cleavage of signal sequences, proteins are secreted by other processes, all together called unconventional. Among the mechanisms operative in unconventional secretion, some are based on two families of extracellular vesicle (EVs), expressed by all types of cells: the exosomes (before secretion called ILVs) and ectosomes (average diameters ∼70 and ∼250 nm). The two types of EVs have been largely characterized by extensive studies. ILVs are assembled within endocytic vacuoles by inward budding of small membrane microdomains associated to cytosolic cargos including unconventional secretory proteins. The vacuoles containing ILVs are called multivesicular bodies (MVBs). Upon their possible molecular exchange with autophagosomes, MVBs undergo two alternative forms of fusion: 1. with lysosomes, followed by large digestion of their cargo molecules; and 2. with plasma membrane (called exocytosis), followed by extracellular diffusion of exosomes. The vesicles of the other type, the ectosomes, are differently assembled. Distinct plasma membrane rafts undergo rapid outward budding accompanied by accumulation of cytosolic/secretory cargo molecules, up to their sewing and pinching off. Both types of EV, released to the extracellular fluid in their complete forms including both membrane and cargo, start navigation for various times and distances, until their fusion with target cells. Release/navigation/fusion of EVs establish continuous tridimensional networks exchanging molecules, signals and information among cells. The proteins unconventionally secreted via EVs are a few hundreds. Some of them are functionally relevant (examples FADD, TNF, TACE), governing physiological processes and important diseases. Such proteins, at present intensely investigated, predict future discoveries and innovative developments, relevant for basic research and clinical practice.

Integrated Strategies for a Holistic View of Extracellular Vesicles

Extracellular vesicles (EVs) are receiving increasing attention for their role in spreading both beneficial and harmful information during cell-cell communication. The complexity and heterogeneity of the origin of EVs make integrated molecular, structural, and functional studies extremely challenging but necessary at the same time. In fact, a comprehensive interdisciplinary approach is needed to correlate the features of EVs, target cells/organs, and the pathophysiological outcomes exerted by the EVs' actions. Based on these premises, after introducing a brief state-of-the-art outline on the current analytical approaches exploited to characterize EVs, this review aims to highlight the effectiveness of those studies where authors put in correlation the diverse EV data collected from different points of view. Although these examples are still just a few, they still represent an excellent starting point to be taken as a reference in the perspective for improving the correlation among EV-related clinical aspects. Of course, to fully reach this goal, several points need to be further improved and developed. Undoubtedly, new avenues in diagnostic, prognostic, and therapeutic applications by EVs will be initiated by integrative strategies, combining biophysical approaches, high-throughput omics technologies, and computational models.

Extracellular vesicle-mediated crosstalk between pancreatic cancer and stromal cells in the tumor microenvironment

Pancreatic ductal adenocarcinoma (PDAC) interacts closely with the tumor microenvironment (TME). The TME is remodeled by crosstalk between pancreatic cancer cells and stromal cells, and is critical for cancer progression. Extracellular vesicles (EVs), including exosomes and microvesicles, help facilitate an exchange of information both within the TME and to distant organs. EVs have also been identified as potential diagnostic biomarkers, therapeutic targets, and drug carriers for pancreatic cancer treatment. Thus, understanding the selective packaging of EVs cargo and its mechanistic impact will increase our understanding of cancer biology. In this review, we collect and analyze recent findings of the pancreatic cancer-stromal cell interactions mediated by EVs and the mechanisms involved in cancer-related immunity and chemoresistance. These studies demonstrate the vital role of EVs in pancreatic cancer reprogramming and TME remodeling. We also summarize the EVs identified as potential PDAC diagnostic biomarkers and possible therapeutic targets. This greater understanding is a promising avenue for transitioning EVs from bench to bedside.

Calorimetry of extracellular vesicles fusion to single phospholipid membrane

Extracellular vesicles (EVs)-mediated communication relies not only on the delivery of complex molecular cargoes as lipids, proteins, genetic material, and metabolites to their target cells but also on the modification of the cell surface local properties induced by the eventual fusion of EVs' membranes with the cells' plasma membrane. Here we applied scanning calorimetry to study the phase transition of single phospholipid (DMPC) monolamellar vesicles, investigating the thermodynamical effects caused by the fusion of doping amounts of mesenchymal stem cells-derived EVs. Specifically, we studied EVs-induced consequences on the lipids distributed in the differently curved membrane leaflets, having different density and order. The effect of EV components was found to be not homogeneous in the two leaflets, the inner (more disordered one) being mainly affected. Fusion resulted in phospholipid membrane flattening associated with lipid ordering, while the transition cooperativity, linked to membrane domains' coexistence during the transition process, was decreased. Our results open new horizons for the investigation of the peculiar effects of EVs of different origins on target cell membrane properties and functionality.