Competition between Bending and Internal Pressure Governs the Mechanics of Fluid Nanovesicles

Encapsulation of single vesicles and single cells in a crosslinked microgel cage

Cell encapsulation provides an efficient strategy to enhance cell durability against harsh external conditions, that offers new possibilities for single-cell applications, such as, tissue engineering and regenerative medicine. Cell encapsulations in hydrogels is developed through various approaches. Still, it remains challenging to achieve single-cell encapsulation where the individual cells are surrounded by a hydrogel layer of well-defined thickness. In this study, temperature-responsive poly(N-isopropylacrylamide)-co-allylamine microgel particles are first assembled into a monolayer at the surface of giant unilamellar lipid vesicles and then inter-microgel crosslinked leading to single-vesicle encapsulation with a pre-defined hydrogel thickness. The same strategy is then extended to yeast cells. The successful encapsulation process is evidenced by the response of the encapsulated lipid vesicles/cells to osmotic gradient, the addition of detergent or salt, as well as changes in temperature. Moreover, cell viability tests show that the hydrogel cage can efficiently protect the cell against external harsh conditions, including elevated temperature, ultraviolet irradiation and osmotic stress. Furthermore, it is demonstrated that the microgel adsorption and interfacial assembly are significantly affected by membrane charge and structural heterogeneity of cell membrane, providing insight into rational design of single-cell encapsulation approach by regulating microgel adsorption on cell membranes with complex composition.

Wrapping nonspherical vesicles at bio-membranes

The wrapping of particles and vesicles by lipid bilayer membranes is a fundamental process in cellular transport and targeted drug delivery. Here, we investigate the wrapping behavior of nonspherical vesicles, such as ellipsoidal, prolate, oblate, and stomatocytes, by systematically varying the bending rigidity of the vesicle membrane and the tension of the initially planar membrane. Using the Helfrich Hamiltonian, triangulated membrane models, and energy minimization techniques, we predict multiple stable-wrapped states and identify the conditions for their coexistence. Our results demonstrate that softer vesicles bind more easily to initially planar membranes; however, complete wrapping requires significantly higher adhesion strength than rigid vesicles. As membrane tension increases, deep-wrapped states disappear at a triple point where shallow-wrapped, deep-wrapped, and complete-wrapped states coexist. The coordinates of the triple point are highly sensitive to the vesicle shape and stiffness. For stomatocytes, increasing stiffness shifts the triple point to higher adhesion strengths and membrane tensions, while for oblates, it shifts to lower values, influenced by shape changes during wrapping. Oblate shapes are preferred in shallow-wrapped states and stomatocytes in deep-wrapped states. In contrast to hard particles, where optimal adhesion strength for complete wrapping occurs at tensionless membranes, complete wrapping of soft vesicles requires finite membrane tension for optimal adhesion strength. These findings provide insights into the interplay between vesicle deformability, shape, and membrane properties, advancing our understanding of endocytosis and the design of advanced biomimetic delivery systems.

Bile Salts Trigger Deformability in Liposomal Vesicles through Edge-Activating Action

This study investigates how bile-salt-based edge activators (EAs) (sodium cholate, NaC; sodium deoxycholate, NaDC; and sodium taurocholate, NaTC) can influence the mechanical properties and deformability of liposomal vesicles. We have elucidated their effect on liposomes composed of l-α-phosphatidylcholine (SPC). Liposomes were formulated using thin-film hydration and characterized using scattering, spectroscopic, and atomic force microscopic (AFM) techniques. Our data show that bile salts can alter the hydrodynamic diameter (Dh), morphology, and mechanical characteristics of vesicles. Their effect on the deformability and Young's modulus of vesicles followed the order NaDC ≥ NaC > NaTC. Breakthrough events were noticed in the vesicles at specific depth levels during force-deformation and force-indentation experiments. Based on the lack of hysteresis in the approach-retract curve, we inferred that the vesicles attained elasticity at lower concentrations of NaDC. Hydrophobic interactions between phospholipids and bile salts were verified from Fourier-transformed infrared spectrophotometer (FTIR) experiments. Increase in bile salt concentration was accompanied by a red shift of the acyl chain (asymmetric stretching CH2 and symmetric stretching CH3) and phosphate groups. This shift suggests enhanced hydrogen bonding between liposomes and bile salts. The affinity of bile salts for the SPC molecule correlated with their relative hydrophobicity. We conclude that NaDC can indeed improve the mechanical properties of liposomes and their ability to penetrate biological barriers.

Unraveling the mysteries of extracellular vesicles with atomic force microscopy: a cutting-edge tool for insights into the microcosm

This review comprehensively explores the remarkable progress in the application of atomic force microscopy (AFM) in extracellular vesicle (EV) research. AFM, with its unique capacity for high-resolution imaging and sensitive force measurement, has arisen as a potent method for studying the nanoscale environment. EVs, a mixed group of membrane vesicles produced by cells, are crucial in facilitating intercellular communication and disease progression. Here, we elucidate the fundamental principles of AFM and the characteristics of EVs. We then delve into the significant findings enabled by the combination of AFM with EV research, including the visualization of EV morphology, the measurement of mechanical properties, and the investigation of interactions with other molecules and cells. Additionally, we discuss the challenges and future directions in this burgeoning field. This review not only highlights the current state-of-the-art but also provides insights into the potential of AFM to further our understanding of EVs and their implications in biology and medicine.

Indocyanine Green Aggregation-Induced Hypotonic Stress to Remodel Aloe Exosome-like Vesicles for Enhanced Tumor Penetration and Phototherapy

As ubiquitous transport nanovesicles in cell biology, plant exosome-like vesicles (PELVs) have enormous potential to deliver drugs safely and effectively. Drug encapsulation and mechanical stability of vesicles are key limitations influencing their delivery efficiency. However, common methods (i.e., ultrasound, electroporation) for drug loading inevitably affect the inherent vesicle characteristics, which influence their stability, leakproof nature, cellular internalization, and tumor penetration. Herein, in order to balance this contradiction, we put forward a strategy to skillfully remodel aloe exosome-like vesicles (AELVs) through indocyanine green (ICG)-induced hypotonic stress during endogenous drug loading. We observe that the rigidity of AELVs is enhanced with the accumulation of long hydrocarbon chain lipids under ICG-induced hypotonic stress. Synchronously, ICG is also loaded into AELVs (ICG/AELVs, IAs), which effectively prevents secondary damage during drug loading. More interestingly, we find that hypotonic stress promotes IA secretion with less intravesicular protein, which is beneficial to enlarge their inner space for more drug loading. The IAs show great storage stability, leakproof, and antidegradation performance. Compared with control AELVs, IAs with higher rigidity are more liable to penetrate into the tumor. IAs further modifying with the AS1411 aptamer (AS1411-IAs, AIAs) exhibit high tumor targeting in vivo. After intravenous administration, the 4T1 tumor is obviously inhibited by AIAs plus NIR irradiation, which effectively improves the survival rate of tumor-bearing mice. Overall, we systematically explore the effects of drug-induced osmotic stress on PELVs during endogenous drug loading and achieve efficient tumor therapy. This work simplifies the process of drug loading in PELVs and enhances their plasticity, which provides a promising perspective for PELV-based drug delivery and clinical application.

Force spectroscopy reveals membrane fluctuations and surface adhesion of extracellular nanovesicles impact their elastic behavior

The elastic properties of nanoscale extracellular vesicles (EVs) are believed to influence their cellular interactions, thus having a profound implication in intercellular communication. However, accurate quantification of their elastic modulus is challenging due to their nanoscale dimensions and their fluid-like lipid bilayer. We show that the previous attempts to develop atomic force microscopy-based protocol are flawed as they lack theoretical underpinning as well as ignore important contributions arising from the surface adhesion forces and membrane fluctuations. We develop a protocol comprising a theoretical framework, experimental technique, and statistical approach to accurately quantify the bending and elastic modulus of EVs. The method reveals that membrane fluctuations play a dominant role even for a single EV. The method is then applied to EVs derived from human embryonic kidney cells and their genetically engineered classes altering the tetraspanin expression. The data show a large spread; the area modulus is in the range of 4 to 19 mN/m and the bending modulus is in the range of 15 to 33 [Formula: see text], respectively. Surprisingly, data for a single EV, revealed by repeated measurements, also show a spread that is attributed to their compositionally heterogeneous fluid membrane and thermal effects. Our protocol uncovers the influence of membrane protein alterations on the elastic modulus of EVs.

Plasmon-Enhanced Fluorescence of Single Extracellular Vesicles Captured in Arrayed Aluminum Nanoholes

Extracellular vesicles (EVs) are nanoparticles encapsulated with a lipid bilayer, and they constitute an excellent source of biomarkers for multiple diseases. However, the heterogeneity in their molecular compositions constitutes a major challenge for their recognition and profiling, thereby limiting their application as an effective biomarker. A single-EV analysis technique is crucial to both the discovery and the detection of EV subpopulations that carry disease-specific signatures. Herein, a plasmonic nanohole array is designed for capturing single EVs and subsequently performing fluorescence detection of their membrane proteins by exploiting plasmonic amplification of the fluorescence signal. Unlike other reported methods, our design relies on an exclusive detection of single EVs captured inside nanoholes, thus allowing us to study only plasmonic effects and avoid other metal-induced phenomena while leveraging on the proximity of emitters to the plasmonic hotspots. The method is optimized through numerical simulations and verified by a combination of atomic force, scanning electron microscopy, and fluorescence microscopy. Fluorescence enhancement is then estimated by measuring the CD9 expression of small EVs derived from the human embryonic kidney (HEK293) cell line and carefully considering the spatial distribution of emission and excitation intensities. Fluorescence intensities of immunostained EVs show a moderate overall enhancement of intensity and follow the intensity trend predicted by simulation for nanohole arrays with different nanohole periods. Moreover, the number of observed EVs in the best-performing nanohole array increases by more than 12 times compared with EVs immobilized on a reference substrate, uncovering a vast amount of weakly fluorescent EVs that would remain undetected with the regular fluorescent method. Our nanohole array provides a basis for a future platform of single-EV analyses, also promising to capture the signature arising from low-expressing proteins.

Atomic force microscopy as a nanomechanical tool for cancer liquid biopsy

Liquid biopsies have been receiving tremendous attention for their potential to reshape cancer management. Though current studies of cancer liquid biopsy primarily focus on applying biochemical assays to characterize the genetic/molecular profiles of circulating tumor cells (CTCs) and their secondary products shed from tumor sites in bodily fluids, delineating the nanomechanical properties of tumor-associated materials in liquid biopsy specimens yields complementary insights into the biology of tumor dissemination and evolution. Particularly, atomic force microscopy (AFM) has become a standard and versatile toolbox for characterizing the mechanical properties of living biological systems at the micro/nanoscale, and AFM has been increasingly utilized to probe the nanomechanical properties of various tumor-derived analytes in liquid biopsies, including CTCs, tumor-associated cells, circulating tumor DNA (ctDNA) molecules, and extracellular vesicles (EVs), offering additional possibilities for understanding cancer pathogenesis from the perspective of mechanobiology. Herein, the applications of AFM in cancer liquid biopsy are summarized, and the challenges and future directions of AFM as a nanomechanical analysis tool in cancer liquid biopsy towards clinical utility are discussed and envisioned.

Broad Adaptability of Coronavirus Adhesion Revealed from the Complementary Surface Affinity of Membrane and Spikes

Coronavirus stands for a large family of viruses characterized by protruding spikes surrounding a lipidic membrane adorned with proteins. The present study explores the adhesion of transmissible gastroenteritis coronavirus (TGEV) particles on a variety of reference solid surfaces that emulate typical virus-surface interactions. Atomic force microscopy informs about trapping effectivity and the shape of the virus envelope on each surface, revealing that the deformation of TGEV particles spans from 20% to 50% in diameter. Given this large deformation range, experimental Langmuir isotherms convey an unexpectedly moderate variation in the adsorption-free energy, indicating a viral adhesion adaptability which goes beyond the membrane. The combination of an extended Helfrich theory and coarse-grained simulations reveals that, in fact, the envelope and the spikes present complementary adsorption affinities. While strong membrane-surface interaction lead to highly deformed TGEV particles, surfaces with strong spike attraction yield smaller deformations with similar or even larger adsorption-free energies.

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.

CD63 sorts cholesterol into endosomes for storage and distribution via exosomes

Extracellular vesicles such as exosomes are now recognized as key players in intercellular communication. Their role is influenced by the specific repertoires of proteins and lipids, which are enriched when they are generated as intraluminal vesicles (ILVs) in multivesicular endosomes. Here we report that a key component of small extracellular vesicles, the tetraspanin CD63, sorts cholesterol to ILVs, generating a pool that can be mobilized by the NPC1/2 complex, and exported via exosomes to recipient cells. In the absence of CD63, cholesterol is retrieved from the endosomes by actin-dependent vesicular transport, placing CD63 and cholesterol at the centre of a balance between inward and outward budding of endomembranes. These results establish CD63 as a lipid-sorting mechanism within endosomes, and show that ILVs and exosomes are alternative providers of cholesterol.

Charge of a transmembrane peptide alters its interaction with lipid membranes

Transmembrane (TM) proteins interact closely with the surrounding membrane lipids. Lipids in the vicinity of TM proteins were reported to have hindered mobility, which has been associated with lipids being caught up in the rough surface of the TM domains. These reports, however, neglect one important factor that largely influences the membrane behavior - electrostatics of the TM peptides that are usually positively charged at their cytosolic end. Here, we study on the example of a neutral and a positively charged WALP peptide, how the charge of a TM peptide influences the membrane. We investigate both its dynamics and mechanics by: (i) time dependent fluorescent shift in combination with classical and FRET generalized polarization to evaluate the mobility of lipids at short and long-range distance from the peptide, (ii) atomic force microscopy to observe the mechanical stability of the peptide-containing membranes, and (iii) molecular dynamics simulations to analyze the peptide-lipid interactions. We show that both TM peptides lower lipid mobility in their closest surroundings. The peptides cause lateral heterogeneity in lipid mobility, which in turn prevents free lipid rearrangement and lowers the membrane ability to seal ruptures after mechanical indentations. Introduction of a positive charge to the peptide largely enhances these effects, affecting the whole membrane. We thus highlight that unspecific peptide-lipid interactions, especially the electrostatics, should not be overlooked as they have a great impact on the mechanics and dynamics of the whole membrane.

Minimal information for studies of extracellular vesicles (MISEV2023): From basic to advanced approaches

Extracellular vesicles (EVs), through their complex cargo, can reflect the state of their cell of origin and change the functions and phenotypes of other cells. These features indicate strong biomarker and therapeutic potential and have generated broad interest, as evidenced by the steady year-on-year increase in the numbers of scientific publications about EVs. Important advances have been made in EV metrology and in understanding and applying EV biology. However, hurdles remain to realising the potential of EVs in domains ranging from basic biology to clinical applications due to challenges in EV nomenclature, separation from non-vesicular extracellular particles, characterisation and functional studies. To address the challenges and opportunities in this rapidly evolving field, the International Society for Extracellular Vesicles (ISEV) updates its 'Minimal Information for Studies of Extracellular Vesicles', which was first published in 2014 and then in 2018 as MISEV2014 and MISEV2018, respectively. The goal of the current document, MISEV2023, is to provide researchers with an updated snapshot of available approaches and their advantages and limitations for production, separation and characterisation of EVs from multiple sources, including cell culture, body fluids and solid tissues. In addition to presenting the latest state of the art in basic principles of EV research, this document also covers advanced techniques and approaches that are currently expanding the boundaries of the field. MISEV2023 also includes new sections on EV release and uptake and a brief discussion of in vivo approaches to study EVs. Compiling feedback from ISEV expert task forces and more than 1000 researchers, this document conveys the current state of EV research to facilitate robust scientific discoveries and move the field forward even more rapidly.

[Atomic Force Microscopy to Measure the Mechanical Property of Nanosized Lipid Vesicles and Its Applications]

Nanoparticles, including liposomes and lipid nanoparticles, have garnered global attention due to their potential applications in pharmaceuticals, vaccines, and gene therapies. These particles enable targeted delivery of new drug modalities such as highly active small molecules and nucleic acids. However, for widespread use of nanoparticle-based formulations, it is crucial to comprehensively analyze their characteristics to ensure both efficacy and safety, as well as enable consistent production. In this context, this review focuses on our research using atomic force microscopy (AFM) to study liposomes and lipid nanoparticles. Our work significantly contributes to the capability of AFM to measure various types of liposomes in an aqueous medium, providing valuable insights into the mechanical properties of these nanoparticles. We discuss the applications of this AFM technique in assessing the quality of nanoparticle-based pharmaceuticals and developing membrane-active peptides.

Proteosomes based on milk phospholipids and proteins to enhance the stability and bioaccessibility of β-carotene

Proteosomes (P) based on milk fat globule membrane's phospholipids (MPs), whey protein isolate (WPI) and sodium caseinate (CasNa) were developed by ultrasonication to encapsulate β-carotene. Entirely milk-ingredients based proteosomes (WPI-MPs-P and CasNa-MPs-P) revealed homogenous distribution with size diameters < 250 nm. WPI-MPs-P depicted positive ζ-potential values (+15.7 ± 0.5 mV), while CasNa-MPs-P demonstrated negative (-32.5 ± 3.4 mV) values of surface charge, respectively and hydrophilic nature of proteosomes was observed by measuring contact-angle (θ). AFM and SEM exhibited spherical to oval and slightly irregular morphology of nanocarriers. For various concentrations of β-carotene, the highest encapsulation efficiency of β-carotene was 90 ± 0.2% and 92 ± 0.8% in WPI-MPs-P and CasNa-MPs-P respectively. FTIR analyses confirmed the hydrophobic and electrostatic interactions-based encapsulation of β-carotene. Beneficial antioxidant-potential of β-carotene was retained after its encapsulation in the proteosomes. Proteosomes increased the digestive-stability (>50%) and bioaccessibility (>85%) of β-carotene. Thus, milk-ingredients based proteosomes offer a novel-strategy to develop functional dairy products to overcome widespread vitamin-A-deficiency.

Mechanical tomography of an archaeal lemon-shaped virus reveals membrane-like fluidity of the capsid and liquid nucleoprotein cargo

Archaeal lemon-shaped viruses have unique helical capsids composed of highly hydrophobic protein strands which can slide past each other resulting in remarkable morphological reorganization. Here, using atomic force microscopy, we explore the biomechanical properties of the lemon-shaped virions of Sulfolobus monocaudavirus 1 (SMV1), a double-stranded DNA virus which infects hyperthermophilic (~80 °C) and acidophilic (pH ~ 2) archaea. Our results reveal that SMV1 virions are extremely soft and withstand repeated extensive deformations, reaching remarkable strains of 80% during multiple cycles of consecutive mechanical assaults, yet showing scarce traces of disruption. SMV1 virions can reversibly collapse wall-to-wall, reducing their volume by ~90%. Beyond revealing the exceptional malleability of the SMV1 protein shell, our data also suggest a fluid-like nucleoprotein cargo which can flow inside the capsid, resisting and accommodating mechanical deformations without further alteration. Our experiments suggest a packing fraction of the virus core to be as low as 11%, with the amount of the accessory proteins almost four times exceeding that of the viral genome. Our findings indicate that SMV1 protein capsid displays biomechanical properties of lipid membranes, which is not found among protein capsids of other viruses. The remarkable malleability and fluidity of the SMV1 virions are likely necessary for the structural transformations during the infection and adaptation to extreme environmental conditions.

Red Blood Cell-Derived Extracellular Vesicles: An Overview of Current Research Progress, Challenges, and Opportunities

Red blood cell-derived extracellular vesicles (RBC EVs) are small, spherical fragments released from red blood cells. These vesicles, similar to EVs derived from other cell types, are crucial for intercellular communication processes and have been implicated in various physiological and pathological processes. The diagnostic and therapeutic potential of RBC EVs has garnered increasing attention in recent years, revealing their valuable role in the field of medicine. In this review, we aim to provide a comprehensive analysis of the current research status of RBC EVs. We summarize existing studies and highlight the progress made in understanding the characteristics and functions of RBC EVs, with a particular focus on their biological roles in different diseases. We also discuss their potential utility as diagnostic and prognostic biomarkers in diseases and as vectors for drug delivery. Furthermore, we emphasize the need for further research to achieve selective purification of RBC EVs and unravel their heterogeneity, which will allow for a deeper understanding of their diverse functions and exploration of their potential applications in diagnostics and therapeutics.

Particle profiling of EV-lipoprotein mixtures by AFM nanomechanical imaging

The widely overlapping physicochemical properties of lipoproteins (LPs) and extracellular vesicles (EVs) represents one of the main obstacles for the isolation and characterization of these pervasive biogenic lipid nanoparticles. We herein present the application of an atomic force microscopy (AFM)-based quantitative morphometry assay to the rapid nanomechanical screening of mixed LPs and EVs samples. The method can determine the diameter and the mechanical stiffness of hundreds of individual nanometric objects within few hours. The obtained diameters are in quantitative accord with those measured via cryo-electron microscopy (cryo-EM); the assignment of specific nanomechanical readout to each object enables the simultaneous discrimination of co-isolated EVs and LPs even if they have overlapping size distributions. EVs and all classes of LPs are shown to be characterised by specific combinations of diameter and stiffness, thus making it possible to estimate their relative abundance in EV/LP mixed samples in terms of stoichiometric ratio, surface area and volume. As a side finding, we show how the mechanical behaviour of specific LP classes is correlated to distinctive structural features revealed by cryo-EM. The described approach is label-free, single-step and relatively quick to perform. Importantly, it can be used to analyse samples which prove very challenging to assess with several established techniques due to ensemble-averaging, low sensibility to small particles, or both, thus providing a very useful tool for quickly assessing the purity of EV/LP isolates including plasma- and serum-derived preparations.

Active buckling of pressurized spherical shells: Monte Carlo simulation

We study the buckling of pressurized spherical shells by Monte Carlo simulations in which the detailed balance is explicitly broken-thereby driving the shell to be active, out of thermal equilibrium. Such a shell typically has either higher (active) or lower (sedate) fluctuations compared to one in thermal equilibrium depending on how the detailed balance is broken. We show that, for the same set of elastic parameters, a shell that is not buckled in thermal equilibrium can be buckled if turned active. Similarly a shell that is buckled in thermal equilibrium can unbuckle if sedated. Based on this result, we suggest that it is possible to experimentally design microscopic elastic shells whose buckling can be optically controlled.

Role of Curvature-Sensing Proteins in the Uptake of Nanoparticles with Different Mechanical Properties

Nanoparticles of different properties, such as size, charge, and rigidity, are used for drug delivery. Upon interaction with the cell membrane, because of their curvature, nanoparticles can bend the lipid bilayer. Recent results show that cellular proteins capable of sensing membrane curvature are involved in nanoparticle uptake; however, no information is yet available on whether nanoparticle mechanical properties also affect their activity. Here liposomes and liposome-coated silica are used as a model system to compare uptake and cell behavior of two nanoparticles of similar size and charge, but different mechanical properties. High-sensitivity flow cytometry, cryo-TEM, and fluorescence correlation spectroscopy confirm lipid deposition on the silica. Atomic force microscopy is used to quantify the deformation of individual nanoparticles at increasing imaging forces, confirming that the two nanoparticles display distinct mechanical properties. Uptake studies in HeLa and A549 cells indicate that liposome uptake is higher than for the liposome-coated silica. RNA interference studies to silence their expression show that different curvature-sensing proteins are involved in the uptake of both nanoparticles in both cell types. These results confirm that curvature-sensing proteins have a role in nanoparticle uptake, which is not restricted to harder nanoparticles, but includes softer nanomaterials commonly used for nanomedicine applications.

Extracellular vesicle-matrix interactions

The extracellular matrix in microenvironments harbors a variety of signals to control cellular functions and the materiality of tissues. Most efforts to synthetically reconstitute the matrix by biomaterial design have focused on decoupling cell-secreted and polymer-based cues. Cells package molecules into nanoscale lipid membrane-bound extracellular vesicles and secrete them. Thus, extracellular vesicles inherently interact with the meshwork of the extracellular matrix. In this Review, we discuss various aspects of extracellular vesicle-matrix interactions. Cells receive feedback from the extracellular matrix and leverage intracellular processes to control the biogenesis of extracellular vesicles. Once secreted, various biomolecular and biophysical factors determine whether extracellular vesicles are locally incorporated into the matrix or transported out of the matrix to be taken up by other cells or deposited into tissues at a distal location. These insights can be utilized to develop engineered biomaterials where EV release and retention can be precisely controlled in host tissue to elicit various biological and therapeutic outcomes.

Non-contact microfluidic analysis of the stiffness of single large extracellular vesicles from IDH1-mutated glioblastoma cells

In preparation for leveraging extracellular vesicles (EVs) for disease diagnostics and therapeutics, fundamental research is being done to understand EV biological, chemical, and physical properties. Most published studies have investigated nanoscale EVs and focused on EV biochemical content. There is much less understanding of large microscale EV characteristics and EV mechanical properties. We recently introduced a non-contact microfluidic technique that measures the stiffness of large EVs (>1 μm diameter). This pilot study probes the robustness of the microfluidic technique to distinguish between EV populations by comparing stiffness distributions of large EVs derived from glioblastoma cell lines. EVs derived from cells expressing the IDH1 mutation, a common glioblastoma mutation known to disrupt lipid metabolism, were stiffer than those expressed from wild-type cells in a statistical comparison of sample medians. A supporting lipidomics analysis showed that the IDH1 mutation increased the amount of saturated lipids in EVs. Taken together, these data encourage further investigation into the potential of high-throughput microfluidics to distinguish between large EV populations that differ in biomolecular composition. These findings contribute to the understanding of EV biomechanics, in particular for the less studied microscale EVs.

Nanomechanical Signatures of Extracellular Vesicles from Hematologic Cancer Patients Unraveled by Atomic Force Microscopy for Liquid Biopsy

Cells release extracellular vesicles (EVs) as the carriers for intercellular communications to regulate life activities. Particularly, it is increasingly apparent that mechanical forces play an essential role in biological systems. The nanomechanical properties of EVs and their dynamics in cancer development are still not fully understood. Herein, with the use of atomic force microscopy (AFM), the nanomechanical signatures of EVs from the liquid biopsies of hematologic cancer patients were unraveled. Single native EVs were probed by AFM under aqueous conditions. The elastic and viscous properties of EVs were measured and visualized to correlate EV mechanics with EV geometry. Experimental results remarkably reveal the significant differences in EV mechanics among multiple myeloma patients, lymphoma patients, and healthy volunteers. The study unveils the unique nanomechanical signatures of EVs in hematologic cancers, which will benefit the studies of liquid biopsies for cancer diagnosis and prognosis with translational significance.

Unraveling How Cholesterol Affects Multivalency-Induced Membrane Deformation of Sub-100 nm Lipid Vesicles

Cholesterol plays a critical role in modulating the lipid membrane properties of biological and biomimetic systems and recent attention has focused on its role in the functions of sub-100 nm lipid vesicles and lipid nanoparticles. These functions often rely on multivalent ligand-receptor interactions involving membrane attachment and dynamic shape transformations while the extent to which cholesterol can influence such interaction processes is largely unknown. To address this question, herein, we investigated the attachment of sub-100 nm lipid vesicles containing varying cholesterol fractions (0-45 mol %) to membrane-mimicking supported lipid bilayer (SLB) platforms. Biotinylated lipids and streptavidin proteins were used as model ligands and receptors, respectively, while the localized surface plasmon resonance sensing technique was employed to track vesicle attachment kinetics in combination with analytical modeling of vesicle shape changes. Across various conditions mimicking low and high multivalency, our findings revealed that cholesterol-containing vesicles could bind to receptor-functionalized membranes but underwent appreciably less multivalency-induced shape deformation than vesicles without cholesterol, which can be explained by a cholesterol-mediated increase in membrane bending rigidity. Interestingly, the extent of vesicle deformation that occurred in response to increasingly strong multivalent interactions was less pronounced for vesicles with greater cholesterol fraction. The latter trend was rationalized by taking into account the strong dependence of the membrane bending energy on the area of the vesicle-SLB contact region and such insights can aid the engineering of membrane-enveloped nanoparticles with tailored biophysical properties.

Lipoprotein particles exhibit distinct mechanical properties

Lipoproteins (LPs) are micelle-like structures with a similar size to extracellular vesicles (EVs) and are therefore often co-isolated, as intensively discussed within the EV community. LPs from human blood plasma are of particular interest as they are responsible for the deposition of cholesterol ester and other fats in the artery, causing lesions, and eventually atherosclerosis. Plasma lipoproteins can be divided according to their size, density and composition into chylomicrons (CM), very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins (HDL). Here, we use atomic force microscopy for mechanical characterization of LPs. We show that the nanoindentation approach used for EV analysis can also be used to characterize LPs, revealing specific differences between some of the particles. Comparing LPs with each other, LDL exhibit a higher bending modulus as compared to CM and VLDL, which is likely related to differences in cholesterol and apolipoproteins. Furthermore, CM typically collapse on the surface after indentation and HDL exhibit a very low height after surface adhesion both being indications for the presence of LPs in an EV sample. Our analysis provides new systematic insights into the mechanical characteristics of LPs.

Hierarchical Quatsome-RGD Nanoarchitectonic Surfaces for Enhanced Integrin-Mediated Cell Adhesion

The synthesis and study of the tripeptide Arg-Gly-Asp (RGD), the binding site of different extracellular matrix proteins, e.g., fibronectin and vitronectin, has allowed the production of a wide range of cell adhesive surfaces. Although the surface density and spacing of the RGD peptide at the nanoscale have already shown a significant influence on cell adhesion, the impact of its hierarchical nanostructure is still rather unexplored. Accordingly, a versatile colloidal system named quatsomes, based on fluid nanovesicles formed by the self-assembling of cholesterol and surfactant molecules, has been devised as a novel template to achieve hierarchical nanostructures of the RGD peptide. To this end, RGD was anchored on the vesicle's fluid membrane of quatsomes, and the RGD-functionalized nanovesicles were covalently anchored to planar gold surfaces, forming a state of quasi-suspension, through a long poly(ethylene glycol) (PEG) chain with a thiol termination. An underlying self-assembled monolayer (SAM) of a shorter PEG was introduced for vesicle stabilization and to avoid unspecific cell adhesion. In comparison with substrates featuring a homogeneous distribution of RGD peptides, the resulting hierarchical nanoarchitectonic dramatically enhanced cell adhesion, despite lower overall RGD molecules on the surface. The new versatile platform was thoroughly characterized using a multitechnique approach, proving its enhanced performance. These findings open new methods for the hierarchical immobilization of biomolecules on surfaces using quatsomes as a robust and novel tissue engineering strategy.

Tools shaping drug discovery and development

Spectroscopic, scattering, and imaging methods play an important role in advancing the study of pharmaceutical and biopharmaceutical therapies. The tools more familiar to scientists within industry and beyond, such as nuclear magnetic resonance and fluorescence spectroscopy, serve two functions: as simple high-throughput techniques for identification and purity analysis, and as potential tools for measuring dynamics and structures of complex biological systems, from proteins and nucleic acids to membranes and nanoparticle delivery systems. With the expansion of commercial small-angle x-ray scattering instruments into the laboratory setting and the accessibility of industrial researchers to small-angle neutron scattering facilities, scattering methods are now used more frequently in the industrial research setting, and probe-less time-resolved small-angle scattering experiments are now able to be conducted to truly probe the mechanism of reactions and the location of individual components in complex model or biological systems. The availability of atomic force microscopes in the past several decades enables measurements that are, in some ways, complementary to the spectroscopic techniques, and wholly orthogonal in others, such as those related to nanomechanics. As therapies have advanced from small molecules to protein biologics and now messenger RNA vaccines, the depth of biophysical knowledge must continue to serve in drug discovery and development to ensure quality of the drug, and the characterization toolbox must be opened up to adapt traditional spectroscopic methods and adopt new techniques for unraveling the complexities of the new modalities. The overview of the biophysical methods in this review is meant to showcase the uses of multiple techniques for different modalities and present recent applications for tackling particularly challenging situations in drug development that can be solved with the aid of fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, atomic force microscopy, and small-angle scattering.

Curvature-based sorting of eight lipid types in asymmetric buckled plasma membrane models

Curvature is a fundamental property of biological membranes and has essential roles in cellular function. Bending of membranes can be induced by their lipid and protein compositions, as well as peripheral proteins, such as those that make up the cytoskeleton. An important aspect of membrane function is the grouping of lipid species into microdomains, or rafts, which serve as platforms for specific biochemical processes. The fluid mosaic model of membranes has evolved to recognize the importance of curvature and leaflet asymmetry, and there are efforts towards evaluating their functional roles. This work investigates the effect of curvature on the sorting of lipids in buckled asymmetric bilayers containing eight lipid types, approximating an average mammalian plasma membrane, through coarse-grained (CG) molecular dynamics (MD) simulations with the Martini force field. The simulations reveal that i) leaflet compositional asymmetry can induce curvature asymmetry, ii) lipids are sorted by curvature to different extents, and iii) curvature-based partitioning trends show moderate to strong correlations with lipid molecular volumes and head to tail bead ratios, respectively. The findings provide unique insights into the role of curvature in membrane organization, and the curvature-based sorting trends should be useful references for later investigations, and potentially interpreting the functional roles of specific lipids.

Effect of hydrophobic moment on membrane interaction and cell penetration of apolipoprotein E-derived arginine-rich amphipathic α-helical peptides

We previously developed an amphipathic arginine-rich peptide, A2-17, which has high ability to directly penetrate across cell membranes. To understand the mechanism of the efficient cell-penetrating ability of the A2-17 peptide, we designed three structural isomers of A2-17 having different values of the hydrophobic moment and compared their membrane interaction and direct cell penetration. Confocal fluorescence microscopy revealed that cell penetration efficiency of peptides tends to increase with their hydrophobic moment, in which A2-17 L14R/R15L, an A2-17 isomer with the highest hydrophobic moment, predominantly remains on plasma cell membranes. Consistently, Trp fluorescence analysis indicated the deepest insertion of A2-17 L14R/R15L into lipid membranes among all A2-17 isomers. Electrophysiological analysis showed that the duration and charge flux of peptide-induced pores in lipid membranes were prominent for A2-17 L14R/R15L, indicating the formation of stable membrane pores. Indeed, the A2-17 L14R/R15L peptide exhibited the strongest membrane damage to CHO-K1 cells. Atomic force microscopy quantitatively defined the peptide-induced membrane perturbation as the decrease in the stiffness of lipid vesicles, which was correlated with the hydrophobic moment of all A2-17 isomers. These results indicate that optimal membrane perturbation by amphipathic A2-17 peptide is critical for its efficient penetration into cells without inducing stabilized membrane pores.

Liposomes and Extracellular Vesicles as Drug Delivery Systems: A Comparison of Composition, Pharmacokinetics, and Functionalization

Over the past decades, lipid-based nanoparticle drug delivery systems (DDS) have caught the attention of researchers worldwide, encouraging the field to rapidly develop improved ways for effective drug delivery. One of the most prominent examples is liposomes, which are spherical shaped artificial vesicles composed of lipid bilayers and able to encapsulate both hydrophilic and hydrophobic materials. At the same time, biological nanoparticles naturally secreted by cells, called extracellular vesicles (EVs), have emerged as promising more complex biocompatible DDS. In this review paper, the differences and similarities in the composition of both vesicles are evaluated, and critical mediators that affect their pharmacokinetics are elucidate. Different strategies that have been assessed to tweak the pharmacokinetics of both liposomes and EVs are explored, detailing the effects on circulation time, targeting capacity, and cytoplasmic delivery of therapeutic cargo. Finally, whether a hybrid system, consisting of a combination of only the critical constituents of both vesicles, could offer the best of both worlds is discussed. Through these topics, novel leads for further research are provided and, more importantly, gain insight in what the liposome field and the EV field can learn from each other.

Dissimilar Deformation of Fluid- and Gel-Phase Liposomes upon Multivalent Interaction with Cell Membrane Mimics Revealed Using Dual-Wavelength Surface Plasmon Resonance

The mechanical properties of biological nanoparticles play a crucial role in their interaction with the cellular membrane, in particular for cellular uptake. This has significant implications for the design of pharmaceutical carrier particles. In this context, liposomes have become increasingly popular, among other reasons due to their customizability and easily varied physicochemical properties. With currently available methods, it is, however, not trivial to characterize the mechanical properties of nanoscopic liposomes especially with respect to the level of deformation induced upon their ligand-receptor-mediated interaction with laterally fluid cellular membranes. Here, we utilize the sensitivity of dual-wavelength surface plasmon resonance to probe the size and shape of bound liposomes (∼100 nm in diameter) as a means to quantify receptor-induced deformation during their interaction with a supported cell membrane mimic. By comparing biotinylated liposomes in gel and fluid phases, we demonstrate that fluid-phase liposomes are more prone to deformation than their gel-phase counterparts upon binding to the cell membrane mimic and that, as expected, the degree of deformation depends on the number of ligand-receptor pairs that are engaged in the multivalent binding.

High-Throughput DNA Tensioner Platform for Interrogating Mechanical Heterogeneity of Single Living Cells

Cell mechanical forces play fundamental roles in regulating cellular responses to environmental stimulations. The shortcomings of conventional methods, including force resolution and cellular throughput, make them less accessible to mechanical heterogeneity at the single-cell level. Here, a DNA tensioner platform is introduced with high throughput (>10 000 cells per chip) and pN-level resolution. A microfluidic-based cell array is trapped on "hairpin-structured" DNA tensioners that enable transformation of the mechanical information of living cells into fluorescence signals. By using the platform, one can identify enhanced mechanical forces of drug-resistant cells as compared to their drug-sensitive counterparts, and mechanical differences between metastatic tumor cells in pleural effusion and nonmetastatic histiocytes. Further genetic analysis traces two genes, VEGFA and MINK1, that may play deterministic roles in regulating mechanical heterogeneities. In view of the ubiquity of cells' mechanical forces in the extracellular microenvironment (ECM), this platform shows wide potential to establish links of cellular mechanical heterogeneity to genetic heterogeneity.

Multivalency-Induced Shape Deformation of Nanoscale Lipid Vesicles: Size-Dependent Membrane Bending Effects

The size of membrane-enveloped virus particles, exosomes, and lipid vesicles strongly impacts functional properties in biological and applied contexts. Multivalent ligand-receptor interactions involving nanoparticle shape deformation are critical to such functions, yet the corresponding effect of nanoparticle size remains largely elusive. Herein, using an indirect nanoplasmonic sensing approach, we investigated how the nanoscale size properties of ligand-modified lipid vesicles affect real-time binding interactions, especially vesicle deformation processes, with a receptor-modified, cell membrane-mimicking platform. Together with theoretical analyses, our findings reveal a pronounced, size-dependent transition in the membrane bending properties of nanoscale lipid vesicles between 60 and 180 nm in diameter. For smaller vesicles, a large membrane bending energy enhanced vesicle stiffness while the osmotic pressure energy was the dominant modulating factor for larger, less stiff vesicles. These findings advance our fundamental understanding of how nanoparticle size affects multivalency-induced nanoparticle shape deformation and can provide guidance for the design of biomimetic nanoparticles with tailored nanomechanical properties.

Single-molecule manipulation of macromolecules on GUV or SUV membranes using optical tweezers

Despite their wide applications in soluble macromolecules, optical tweezers have rarely been used to characterize the dynamics of membrane proteins, mainly due to the lack of model membranes compatible with optical trapping. Here, we examined optical trapping and mechanical properties of two potential model membranes, giant and small unilamellar vesicles (GUVs and SUVs, respectively) for studies of membrane protein dynamics. We found that optical tweezers can stably trap GUVs containing iodixanol with controlled membrane tension. The trapped GUVs with high membrane tension can serve as a force sensor to accurately detect reversible folding of a DNA hairpin or membrane binding of synaptotagmin-1 C2AB domain attached to the GUV. We also observed that SUVs are rigid enough to resist large pulling forces and are suitable for detecting protein conformational changes induced by force. Our methodologies may facilitate single-molecule manipulation studies of membrane proteins using optical tweezers.

Extracellular vesicles as delivery systems at nano-/micro-scale

Extracellular vesicles (EVs) have shown significant promises as nano-/micro-size carriers in drug delivery and bioimaging. With more characteristics of EVs explored through tremendous research efforts, their unmatched physicochemical properties, biological features, and mechanical aspects make them unique vehicles, owning exceptional pharmacokinetics, circulatory metabolism and biodistribution pattern when delivering theranostic cargoes. In this review we firstly analyzed pros and cons of the EVs as a delivery platform. Secondly, compared to engineered nanoparticle delivery systems, such as biocompatible di-block co-polymers, rational design to improve EVs (exosomes in particular) were elaborated. Lastly, different pharmaceutical loading approaches into EVs were compared, reaching a conclusion on how to construct a clinically available and effective nano-/micro-carrier for a satisfactory medical mission.

Stiffness of Fluid and Gel Phase Lipid Nanovesicles: Weighting the Contributions of Membrane Bending Modulus and Luminal Pressurization

The mechanical properties of biogenic membranous compartments are thought to be relevant in numerous biological processes; however, their quantitative measurement remains challenging for most of the already available force spectroscopy (FS)-based techniques. In particular, the debate on the mechanics of lipid nanovesicles and on the interpretation of their mechanical response to an applied force is still open. This is mostly due to the current lack of a unified model being able to describe the mechanical response of both gel and fluid phase lipid vesicles and to disentangle the contributions of membrane rigidity and luminal pressure. In this framework, we herein propose a simple model in which the interplay of membrane rigidity and luminal pressure to the overall vesicle stiffness is described as a series of springs; this approach allows estimating these two contributions for both gel and fluid phase liposomes. Atomic force microscopy-based FS, performed on both vesicles and supported lipid bilayers, is exploited for obtaining all the parameters involved in the model. Moreover, the use of coarse-grained full-scale molecular dynamics simulations allowed for better understanding of the differences in the mechanical responses of gel and fluid phase bilayers and supported the experimental findings. The results suggest that the pressure contribution is similar among all the probed vesicle types; however, it plays a dominant role in the mechanical response of lipid nanovesicles presenting a fluid phase membrane, while its contribution becomes comparable to the one of membrane rigidity in nanovesicles with a gel phase lipid membrane. The results presented herein offer a simple way to quantify two of the most important parameters in vesicle nanomechanics (membrane rigidity and internal pressurization), and as such represent a first step toward a currently unavailable, unified model for the mechanical response of gel and fluid phase lipid nanovesicles.

Quantitative Nanomechanical Analysis of Small Extracellular Vesicles for Tumor Malignancy Indication

The nanomechanical properties of tumor-derived small extracellular vesicles (sEVs) are essential to cancer progression. Here, nanoindentation is utilized on atomic force microscopy (AFM) to quantitatively investigate the nanomechanical properties of human breast cancer cell-derived sEVs at single vesicle level and explore their relationship with tumor malignancy and vesicle size. It is demonstrated that the stiffness of the sEVs results from the combined contribution of the bending modulus and osmotic pressure of the sEVs. The stiffness and osmotic pressure increase with increasing malignancy of the sEVs and decrease with increasing size of the sEVs. The bending modulus decreases with increasing malignancy of the sEVs and is lower in smaller sEVs. This study builds relationship between the nanomechanical signature of the sEV and tumor malignancy, adding information for better understanding cancer mechanobiology.

Structural and mechanical characteristics of exosomes from osteosarcoma cells explored by 3D-atomic force microscopy

Exosomes have recently gained interest as mediators of cell-to-cell communication and as potential biomarkers for cancer and other diseases. They also have potential as nanocarriers for drug delivery systems. Therefore, detailed structural, molecular, and biomechanical characterization of exosomes is of great importance for developing methods to detect and identify the changes associated with the presence of cancer and other diseases. Here, we employed three-dimensional atomic force microscopy (3D-AFM) to reveal the structural and nanomechanical properties of exosomes at high spatial resolution in physiologically relevant conditions. The substructural details of exosomes released from three different cell types were determined based on 3D-AFM force mapping. The resulting analysis revealed the presence of distinct local domains bulging out from the exosome surfaces, which were associated with the exosomal membrane proteins present on the outer surface. The nanomechanical properties of individual exosomes were determined from the 3D-force maps. We found a considerably high elastic modulus, ranging from 50 to 350 MPa, as compared to that obtained for synthetic liposomes. Moreover, malignancy-dependent changes in the exosome mechanical properties were revealed by comparing metastatic and nonmetastatic tumor cell-derived exosomes. We found a clear difference in their Young's modulus values, suggesting differences in their protein profiles and other exosomal contents. Exosomes derived from a highly aggressive and metastatic k-ras-activated human osteosarcoma (OS) cell line (143B) showed a higher Young's modulus than that derived from a nonaggressive and nonmetastatic k-ras-wildtype human OS cell line (HOS). The increased elastic modulus of the 143B cell-derived exosomes was ascribed to the presence of abundant specific proteins responsible for elastic fiber formation as determined by mass spectroscopy and confirmed by western blotting and ELISA. Therefore, we conclude that exosomes derived from metastatic tumor cells carry an exclusive protein content that differs from their nonmetastatic counterparts, and thus they exhibit different mechanical characteristics. Discrimination between metastatic and nonmetastatic malignant cell-derived exosomes would be of great importance for studying exosome biological functions and using them as diagnostic biomarkers for various tumor types. Our findings further suggest that metastatic tumor cells release exosomes that express increased levels of elastic fiber-associated proteins to preserve their softness.

Elastic moduli of lipid membranes: Reproducibility of AFM measures

Membrane elastic properties play a major role in membrane remodeling events, such as vesicle fusion and fission. They are also crucial in drug delivery by liposomes. Different experimental techniques are available to measure elastic properties. Among them, atomic force microscopy (AFM) presents the unique advantage of being directly applicable to nano-sized liposomes. Unfortunately, different AFM measures reported in the literature show little agreement among each other and are difficult to compare with measures of bending modulus obtained by other experimental techniques or by molecular simulations. In this work we determine the bending rigidity of Egg PC liposomes in terms of Young modulus via AFM measurements, using two different tip shapes and different cantilever force constants. We interpret the measures using the Hertz and Shell models, and observe a clear dependency of the Young modulus values on the tip properties and on the interpretative theory. The effect of the AFM tip shape is less important than the effect of the cantilever force constant, and the mathematical model has a major effect on the interpretation of the data. The Shell theory provides the closest agreement between AFM data and other experimental data for the membrane bending modulus. Finally, we compare the results to calculations of bending modulus from molecular dynamics simulations of membrane buckles. Simulations provide values of bending modulus consistent with literature data, but the agreement with AFM experiments is reasonable only for some specific experimental conditions.

Physicochemical Characterization of Liposomes That Mimic the Lipid Composition of Exosomes for Effective Intracellular Trafficking

Exosomes mediate communication between cells in the body by the incorporation and transfer of biological materials. To design an artificial liposome, which would mimic the lipid composition and physicochemical characteristics of naturally occurring exosomes, we first studied the physicochemical properties of exosomes secreted from HepG2 cells. The exosome stiffness obtained by atomic force microscopy was moderate. Some liposomes were then fabricated to mimic the representative reported lipid composition of exosomes. Their physicochemical properties and cellular internalization efficiencies were investigated to optimize the cellular internalization efficiency of the liposomes. A favorable internalization efficiency was obtained by incubating HeLa cells with 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)/cholesterol (Chol)/1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPS) (40/40/20 mol %) liposomes, which have a similar stiffness and zeta potential to exosomes. A dramatic increase in internalization efficiency was demonstrated by adding DOPS to simple DSPC/Chol liposomes. We found that DOPS had a more desirable effect on cellular internalization than its saturated lipid counterpart, 1,2-distearoyl-sn-glycero-3-phospho-l-serine. Furthermore, it was shown that the phosphatidylserine-binding protein, T-cell immunoglobulin mucin protein 4, was largely involved in the intracellular transfer of DSPC/Chol/DOPS liposomes. Thus, DOPS was a key lipid to provide the appropriate stiffness, zeta potential, and membrane surface affinity of the resulting liposome. Our results may help develop efficient drug carriers aiming to internalize active substances into cells.

Detection of material-derived differences in the stiffness of egg yolk phosphatidylcholine-containing liposomes using atomic force microscopy

Naturally sourced phospholipids are used in many liposomal pharmaceuticals. The present report describes a method to detect the effects of different egg yolk phosphatidylcholines (EPCs) on liposomal physicochemical properties. Five EPC-containing liposomes were prepared using five different EPCs obtained from different suppliers. There was no significant difference in purity between each EPC. The stiffness of the liposomes was examined via atomic force microscopy (AFM) in relation to the liposomal membrane permeability coefficient of encapsulated calcein after gel filtration, which is indicative of liposomal stability including the release of a hydrophilic drug from a liposome. Although the size of the liposome and the encapsulation efficiency of calcein did not significantly change with the type of EPC used, the liposome stiffness was found to vary depending on the EPC used, and liposomes with a similar stiffness were found to show a similar membrane permeability to calcein. Our results indicate the usefulness of stiffness measurement, using AFM as the analytical method, to detect material-derived differences in EPC-containing liposomes that affect drug release from the liposomes. Because drug release is one of the most important liposomal functions, combining this method with other analytical methods could be useful in selecting material for the development and quality control of EPC-containing liposomes.

Stiffness of HIV-1 Mimicking Polymer Nanoparticles Modulates Ganglioside-Mediated Cellular Uptake and Trafficking

The monosialodihexosylganglioside, GM3, and its binding to CD169 (Siglec-1) have been indicated as key factors in the glycoprotein-independent sequestration of the human immunodeficiency virus-1 (HIV-1) in virus-containing compartments (VCCs) in myeloid cells. Here, lipid-wrapped polymer nanoparticles (NPs) are applied as a virus-mimicking model to characterize the effect of core stiffness on NP uptake and intracellular fate triggered by GM3-CD169 binding in macrophages. GM3-functionalized lipid-wrapped NPs are assembled with poly(lactic-co-glycolic) acid (PLGA) as well as with low and high molecular weight polylactic acid (PLAlMW and PLAhMW) cores. The NPs have an average diameter of 146 ± 17 nm and comparable surface properties defined by the self-assembled lipid layer. Due to differences in the glass transition temperature, the Young's modulus (E) differs substantially under physiological conditions between PLGA (E PLGA = 60 ± 32 MPa), PLAlMW (E PLA lMW = 86 ± 25 MPa), and PLAhMW (E PLA hMW = 1.41 ± 0.67 GPa) NPs. Only the stiff GM3-presenting PLAhMW NPs but not the softer PLGA or PLAlMW NPs avoid a lysosomal pathway and localize in tetraspanin (CD9)-positive compartments that resemble VCCs. These observations suggest that GM3-CD169-induced sequestration of NPs in nonlysosomal compartments is not entirely determined by ligand-receptor interactions but also depends on core stiffness.

Independent Size and Fluorescence Emission Determination of Individual Biological Nanoparticles Reveals that Lipophilic Dye Incorporation Does Not Scale with Particle Size

Advancements in nanoparticle characterization techniques are critical for improving the understanding of how biological nanoparticles (BNPs) contribute to different cellular processes, such as cellular communication, viral infection, as well as various drug-delivery applications. Since BNPs are intrinsically heterogeneous, there is a need for characterization methods that are capable of providing information about multiple parameters simultaneously, preferably at the single-nanoparticle level. In this work, fluorescence microscopy was combined with surface-based two-dimensional flow nanometry, allowing for simultaneous and independent determination of size and fluorescence emission of individual BNPs. In this way, the dependence of the fluorescence emission of the commonly used self-inserting lipophilic dye 3,3'-dioctadecyl-5,5'-di(4-sulfophenyl)oxacarbocyanine (SP-DiO) could successfully be correlated with nanoparticle size for different types of BNPs, including synthetic lipid vesicles, lipid vesicles derived from cellular membrane extracts, and extracellular vesicles derived from human SH-SY5Y cell cultures; all vesicles had a radius, r, of ∼50 nm and similar size distributions. The results demonstrate that the dependence of fluorescence emission of SP-DiO on nanoparticle size varies significantly between the different types of BNPs, with the expected dependence on membrane area, r², being observed for synthetic lipid vesicles, while a significant weaker dependence on size was observed for BNPs with more complex composition. The latter observation is attributed to a size-dependent difference in membrane composition, which may influence either the optical properties of the dye and/or the insertion efficiency, indicating that the fluorescence emission of this type of self-inserting dye may not be reliable for determining size or size distribution of BNPs with complex lipid compositions.

Impact of isolation methods on the biophysical heterogeneity of single extracellular vesicles

Extracellular vesicles (EVs) have raised high expectations as a novel class of diagnostics and therapeutics. However, variabilities in EV isolation methods and the unresolved structural complexity of these biological-nanoparticles (sub-100 nm) necessitate rigorous biophysical characterization of single EVs. Here, using atomic force microscopy (AFM) in conjunction with direct stochastic optical reconstruction microscopy (dSTORM), micro-fluidic resistive pore sizing (MRPS), and multi-angle light scattering (MALS) techniques, we compared the size, structure and unique surface properties of breast cancer cell-derived small EVs (sEV) obtained using four different isolation methods. AFM and dSTORM particle size distributions showed coherent unimodal and bimodal particle size populations isolated via centrifugation and immune-affinity methods respectively. More importantly, AFM imaging revealed striking differences in sEV nanoscale morphology, surface nano-roughness, and relative abundance of non-vesicles among different isolation methods. Precipitation-based isolation method exhibited the highest particle counts, yet nanoscale imaging revealed the additional presence of aggregates and polymeric residues. Together, our findings demonstrate the significance of orthogonal label-free surface characteristics of single sEVs, not discernable via conventional particle sizing and counts alone. Quantifying key nanoscale structural characteristics of sEVs, collectively termed ‘EV-nano-metrics’ enhances the understanding of the complexity and heterogeneity of sEV isolates, with broad implications for EV-analyte based research and clinical use.

Mechanical Characterization of Liposomes and Extracellular Vesicles, a Protocol

Both natural as well as artificial vesicles are of tremendous interest in biology and nanomedicine. Small vesicles (<200 nm) perform essential functions in cell biology and artificial vesicles (liposomes) are used as drug delivery vehicles. Atomic Force Microscopy (AFM) is a powerful technique to study the structural properties of these vesicles. AFM is a well-established technique for imaging at nanometer resolution and for mechanical measurements under physiological conditions. Here, we describe the procedure of AFM imaging and force spectroscopy on small vesicles. We discuss how to image vesicles with minimal structural disturbance, and how to analyze the data for accurate size and shape measurements. In addition, we describe the procedure for performing nanoindentations on vesicles and the subsequent data analysis including mechanical models used for data interpretation.

Ligand-receptor-mediated attachment of lipid vesicles to a supported lipid bilayer

The interaction of exosomes (cell-secreted [Formula: see text]100 nm-sized extracellular vesicles) or membrane-enveloped virions with cellular lipid membranes is often mediated by relatively weak ligand-receptor bonds. Interactions of this type can be studied using vesicles and observing their attachment to receptors located in a lipid bilayer formed at a solid surface. The contact region between a vesicle and the supported lipid bilayer and accordingly the number of ligand-receptor pairs there can be increased by deforming a vesicle. Herein, I (i) estimate theoretically the corresponding deformation energy assuming a disk-like or elongated shape of vesicles, (ii) present the equations allowing one to track such deformations by employing total internal reflection fluorescence microscopy and surface plasmon resonance, and (iii) briefly discuss some related experimental studies.

Non-contact microfluidic mechanical property measurements of single apoptotic bodies

BACKGROUND

Cells exchange information by secreting micro- and nanosized extracellular vesicles (EVs), ranging from exosomes (30-100 nm) to apoptotic bodies (ABs, 1-5 μm). There is still much to understand about fundamental EV biological, physical, and chemical properties before clinical applications can be developed. EV mechanical properties have only been measured with atomic force microscopy (AFM) with its problematic adhesion and hard substrate effects. To understand EV mechanical behavior in less extreme mechanical conditions relevant to blood flow and many soft tissue environments, a non-contact measurement technique is needed.

METHODS

We measured the mechanical properties of single microscale ABs derived from human blood plasma using non-contact microfluidics. EVs were gently stretched in extensional flow, similar to a traditional tensile test, and a linear mechanical model was applied to estimate mechanical stiffnesses from the observed stretching.

RESULTS

The effective shear elastic modulus of ABs in non-contact flow conditions is approximately 5.6 ± 0.5 Pa, 7 orders of magnitude lower than previously reported AFM-measured biological exosome stiffnesses and 200 times smaller than suspended cells.

CONCLUSIONS

Apoptotic bodies are very soft in fluid environments and exhibit lower effective stiffnesses than suspended cells. By measuring ABs in a natural fluid environment and low-force regime without hard probes and surfaces, we achieved closer agreement with linear mechanical theory and therefore more accurate stiffness measurements.

GENERAL SIGNIFICANCE

AFM manufacturers and users should consider implementing new mechanical models to interpret AFM force indentation curves so that accurate extracellular vesicle mechanical properties can be extracted.