Trypan Blue - Adapting a Dye Used for Labelling Dead Cells to Visualize Pinocytosis in Viable Cells.

BACKGROUND/AIMS: Trypan blue is routinely used in cell culture experiments to distinguish between dead cells, which are labelled by trypan blue, and viable cells, which are apparently free of any staining. The assumption that trypan blue labelling is restricted to dead cells derives from the observation that rupture of the plasma membrane correlates with intense trypan blue staining. However, decades ago, trypan blue has been used to trace fluid uptake by viable macrophage-like cells in animals. These studies contributed to the concept of the reticuloendothelial system in vertebrates. Trypan blue itself does not show a fluorescence signal, but trypan blue-labelled proteins do. Therefore, intracellular localization of trypan blue-labelled proteins could give a clue to the entrance pathway of the dye in viable cells. METHODS: We used fluorescence microscopy to visualize trypan blue positive structures and to evaluate whether the bactericide, silver, enhances cellular trypan blue uptake in the brain macrophage-like cell line, BV-2. The pattern of chromatin condensation, visualized by DAPI staining, was used to identify the cell death pathway. RESULTS: We observed that silver nitrate at elevated concentrations (≥ 10 µM) induced in most cells a necrotic cell death pathway. Necrotic cells, identified by pycnotic nuclei, showed an intense and homogenous trypan blue staining. Apoptotic cells, characterized by crescent-like nuclear chromatin condensations, were not labelled by trypan blue. At lower silver nitrate concentrations, most cells were viable, but they showed trypan blue labelling. Viable cells showed a cell-type specific distribution of heterochromatin and revealed a perinuclear accumulation of bright trypan blue-labelled vesicles and, occasionally, a faint homogenous trypan blue labelling of the cytoplasm and nucleus. Amiloride, which prevents macropinocytosis by blocking the Na+ / H+ exchange, suppressed perinuclear accumulation of dye-labelled vesicles. Swelling of cells in a hypotonic solution induced an intense intracellular accumulation of trypan blue. Cells exposed to a hypotonic solution in the presence of 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), which blocks volume-regulated ion channels, prevented labelling of the cytoplasm and nucleus but did not affect labelling of perinuclear vesicles. CONCLUSION: In viable cells trypan blue-labelled vesicles indicate trypan blue uptake by macropinocytosis and trypan blue-labelled cytosol could indicate a further entry pathway for the dye, like activated volume-regulated channels. Accordingly, fluorescence microscopic analysis of trypan blue-labelled cells allows not only a discrimination between necrotic and apoptotic cell death pathway but also a discrimination between the mode of trypan blue uptake in viable cells - via pinocytosis or via activated volume-regulated ion channels - in the same preparation at the single cell level.

High efficiency preparation of monodisperse plasma membrane derived extracellular vesicles for therapeutic applications.

Extracellular vesicles (EVs) are highly interesting for the design of next-generation therapeutics. However, their preparation methods face challenges in standardization, yield, and reproducibility. Here, we describe a highly efficient and reproducible EV preparation method for monodisperse nano plasma membrane vesicles (nPMVs), which yields 10 to 100 times more particles per cell and hour than conventional EV preparation methods. nPMVs are produced by homogenizing giant plasma membrane vesicles following cell membrane blebbing and apoptotic body secretion induced by chemical stressors. nPMVs showed no significant differences compared to native EVs from the same cell line in cryo-TEM analysis, in vitro cellular interactions, and in vivo biodistribution studies in zebrafish larvae. Proteomics and lipidomics, on the other hand, suggested substantial differences consistent with the divergent origin of these two EV types and indicated that nPMVs primarily derive from apoptotic extracellular vesicles. nPMVs may provide an attractive source for developing EV-based pharmaceutical therapeutics.

Surface display of functional moieties on extracellular vesicles using lipid anchors.

Extracellular vesicles (EVs) are efficient natural vehicles for intercellular communication and are under extensive investigation for the delivery of diverse therapeutics including small molecule drugs, nucleic acids, and proteins. To understand the mechanisms behind the biological activities of EVs and develop EV therapeutics, it's fundamental to track EVs and engineer EVs in a customized manner. In this study, we identified, using single-vesicle flow cytometry and microscopy, the lipid DOPE (dioleoyl phosphatidyl ethanolamine) as an efficient anchor for isolated EVs. Notably, DOPE associated with EVs quickly, and the products remained stable under several challenging conditions. Moreover, conjugating fluorophores, receptor-targeting peptides or albumin-binding molecules with DOPE enabled tracking the cellular uptake, enhanceing the cellular uptake or extending the circulation time in mice of engineered EVs , respectively. Taken together, this study reports an efficient lipid anchor for exogenous engineering of EVs and further showcases its versatility for the functionalization of EVs.

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.

EVAnalyzer: High content imaging for rigorous characterisation of single extracellular vesicles using standard laboratory equipment and a new open-source ImageJ/Fiji plugin.

Extracellular vesicle (EV) research increasingly demands for quantitative characterisation at the single vesicle level to address heterogeneity and complexity of EV subpopulations. Emerging, commercialised technologies for single EV analysis based on, for example, imaging flow cytometry or imaging after capture on chips generally require dedicated instrumentation and proprietary software not readily accessible to every lab. This limits their implementation for routine EV characterisation in the rapidly growing EV field. We and others have shown that single vesicles can be detected as light diffraction limited fluorescent spots using standard confocal and widefield fluorescence microscopes. Advancing this simple strategy into a process for routine EV quantitation, we developed 'EVAnalyzer', an ImageJ/Fiji (Fiji is just ImageJ) plugin for automated, quantitative single vesicle analysis from imaging data. Using EVAnalyzer, we established a robust protocol for capture, (immuno-)labelling and fluorescent imaging of EVs. To exemplify the application scope, the process was optimised and systematically tested for (i) quantification of EV subpopulations, (ii) validation of EV labelling reagents, (iii) in situ determination of antibody specificity, sensitivity and species cross-reactivity for EV markers and (iv) optimisation of genetic EV engineering. Additionally, we show that the process can be applied to synthetic nanoparticles, allowing to determine siRNA encapsulation efficiencies of lipid-based nanoparticles (LNPs) and protein loading of SiO2 nanoparticles. EVAnalyzer further provides a pipeline for automated quantification of cell uptake at the single cell-single vesicle level, thereby enabling high content EV cell uptake assays and plate-based screens. Notably, the entire procedure from sample preparation to the final data output is entirely based on standard reagents, materials, laboratory equipment and open access software. In summary, we show that EVAnalyzer enables rigorous characterisation of EVs with generally accessible tools. Since we further provide the plugin as open-source code, we expect EVAnalyzer to not only be a resource of immediate impact, but an open innovation platform for the EV and nanoparticle research communities.

Genotoxicity risk assessment in fish (Rutilus rutilus) from two contaminated rivers in the Kosovo.

Being exposed to untreated urban and industrial water, the rivers Drenica and Sitnica are considered to be the most polluted ones in the Kosovo. Our previous investigations on the cyto- and genotoxic potential of water samples from these rivers evaluated with primary rat hepatocyte cultures indicated a risk for the health of aquatic organisms. In order to assess the genotoxic risk to aquatic organisms, we therefore performed a two year study (2016-2017) on roach (Rutilus rutilus) from these rivers. Specimens were collected at three locations along the Drenica river and two locations along the Sitnica river, and the genotoxicity was evaluated by the micronucleus as well as the Comet assay (DNA damage) in erythrocytes. The frequencies of micronucleated cells were determined for samples collected in four seasons, whereas the Comet assay was employed on samples collected in five seasons during the two-year period. The data obtained revealed an increase of the frequency of micronucleated erythrocytes from Rutilus rutilus collected at most sampling locations and from both rivers at all seasons investigated. Significant differences to the control (lake Badovc) were found in summer 2016 and spring 2017 samples. When comparing the seasons, the summer 2016 samples were most genotoxic, followed by spring 2017 and autumn 2016. With regard to the Comet assay data, a similar but more prominent "response" was observed. Another important observation is that micronucleus rates as well as DNA damage levels were significantly higher in samples collected in 2016 compared to the respective seasons in 2017. Altogether, the "response" obtained with both markers confirmed a genotoxic risk for fish due the pollution of these rivers. Since there were, however, seasonal and annual variations of the genotoxicity levels further in depth studies have to be carried out addressing the nature of these changes.

Ferritin-stimulated lipid peroxidation, lysosomal leak, and macroautophagy promote lysosomal "metastability" in primary hepatocytes determining in vitro cell survival.

Several pathologies are associated with elevated levels of serum ferritin, for which growth inhibitory properties have been reported; however, the underlying mechanisms are still poorly defined. Previously we have described cytotoxic properties of isoferritins released from primary hepatocytes in vitro, which induce apoptosis in an iron and oxidative stress-dependent mode. Here we show that this ferritin species stimulates endosome clustering and giant endosome formation in primary hepatocytes accompanied by enhanced lysosomal membrane permeability (LMP). In parallel, protein modification by lipid peroxidation-derived 4-hydroxynonenal (HNE) is strongly promoted by ferritin, the HNE-modified proteins (HNE-P) showing remarkable aggregation. Emphasizing the prooxidant context, GSH is rapidly depleted and the GSH/GSSG ratio is substantially declining in ferritin-treated cells. Furthermore, ferritin triggers a transient upregulation of macroautophagy which is abolished by iron chelation and apparently supports HNE-P clearance. Macroautophagy inhibition by 3-methyladenine strongly amplifies ferritin cytotoxicity in a time- and concentration-dependent mode, suggesting an important role of macroautophagy on cellular responses to ferritin endocytosis. Moreover, pointing at an involvement of lysosomal proteolysis, ferritin cytotoxicity and lysosome fragility are aggravated by the protease inhibitor leupeptin. In contrast, EGF which suppresses ferritin-induced cell death attenuates ferritin-mediated LMP. In conclusion, we propose that HNE-P accumulation, lysosome dysfunction, and macroautophagy stimulated by ferritin endocytosis provoke lysosomal "metastability" in primary hepatocytes which permits cell survival as long as in- and extrinsic determinants (e.g., antioxidant availability, damage repair, EGF signaling) keep the degree of lysosomal destabilization below cell death-inducing thresholds.