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A Platform for Actively Loading Cargo RNA to Elucidate Limiting Steps in EV-mediated Delivery

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A Platform for Actively Loading Cargo RNA to Elucidate Limiting Steps in EV-mediated Delivery

Michelle E Hung et al. J Extracell Vesicles.

Abstract

Extracellular vesicles (EVs) mediate intercellular communication through transfer of RNA and protein between cells. Thus, understanding how cargo molecules are loaded and delivered by EVs is of central importance for elucidating the biological roles of EVs and developing EV-based therapeutics. While some motifs modulating the loading of biomolecular cargo into EVs have been elucidated, the general rules governing cargo loading and delivery remain poorly understood. To investigate how general biophysical properties impact loading and delivery of RNA by EVs, we developed a platform for actively loading engineered cargo RNAs into EVs. In our system, the MS2 bacteriophage coat protein was fused to EV-associated proteins, and the cognate MS2 stem loop was engineered into cargo RNAs. Using this Targeted and Modular EV Loading (TAMEL) approach, we identified a configuration that substantially enhanced cargo RNA loading (up to 6-fold) into EVs. When applied to vesicles expressing the vesicular stomatitis virus glycoprotein (VSVG) - gesicles - we observed a 40-fold enrichment in cargo RNA loading. While active loading of mRNA-length (>1.5 kb) cargo molecules was possible, active loading was much more efficient for smaller (~0.5 kb) RNA molecules. We next leveraged the TAMEL platform to elucidate the limiting steps in EV-mediated delivery of mRNA and protein to prostate cancer cells, as a model system. Overall, most cargo was rapidly degraded in recipient cells, despite high EV-loading efficiencies and substantial EV uptake by recipient cells. While gesicles were efficiently internalized via a VSVG-mediated mechanism, most cargo molecules were rapidly degraded. Thus, in this model system, inefficient endosomal fusion or escape likely represents a limiting barrier to EV-mediated transfer. Altogether, the TAMEL platform enabled a comparative analysis elucidating a key opportunity for enhancing EV-mediated delivery to prostate cancer cells, and this technology should be of general utility for investigations and applications of EV-mediated transfer in other systems.

Keywords: CD63; Lamp2b; MS2 coat protein dimer; VSVG; active loading; exosomes; extracellular vesicles; microvesicles.

Figures

Fig. 1
Fig. 1
Evaluation of RNA loading into EVs via the TAMEL platform. (a) This cartoon summarizes the concept of facilitating active loading of cargo RNA into EVs via our TAMEL platform. A TAMEL EV-loading protein comprises an EV-enriched protein (EEP, blue) fused to an RNA-binding domain (RBD, green), which localizes to EVs. Actively loaded RNA (green) contains a motif that binds to the RBD, resulting in enhanced loading into EVs relative to passively loaded RNA (orange). (b) RNA cargo design impacts active loading. The “fold enrichment of cargo mRNA +/−MS2” is defined as the ratio of cargo RNA/GAPDH mRNA in EVs derived from cells expressing Lamp2b–MS2–HA divided by the same RNA ratio in EVs derived from cells expressing Lamp2b–HA. All experiments were performed in biological triplicates. (c) Cartoon illustrating the 3′ RNA fragment analysis technique. Cargo RNA is first reverse transcribed using an oligo dT primer, and amplicons corresponding to the RNA 5′ or 3′ ends (the latter is located ~500 bases upstream of the polyA site) are then quantified by qPCR using the primer pairs indicated. Note that the amplicon near the RNA 3′ end will be present in cDNA derived from both full-length RNA and 3′ RNA fragments. (d) Analysis of 3′ RNA fragment loading into EVs. Cargo RNA levels were quantified as depicted in panel c and normalized to GAPDH. Passive loading: cells transfected with Lamp2b–HA; active loading: cells transfected with Lamp2b–MS2–HA. (e) Full-length RNA and 3′ fragment RNA levels in EVs were quantified following incubation at 37°C; experiments were performed in technical duplicate with a biological replicate shown in Supplementary Fig. 2a. Error bars indicate 1 standard deviation, throughout. MVB, multivesicular body.
Fig. 2
Fig. 2
Impact of EEP choice on TAMEL-mediated active RNA loading into vesicles. (a) Effects of EEP choice on cargo RNA loading into EVs. Experiments were performed in biological triplicate. (b) Effects of EEP choice on cargo loading into gesicles. Experiments were performed in biological triplicate. (c) Protein abundance was quantified by densitometry analysis of anti-HA western blots (Supplementary Fig. 3), and each blot was internally normalized by the intensity for VSVG–MS2–HA in gesicles (maximal intensity case). This experiment was performed in biological duplicate. The y-axis is in log scale to enable visualization of all values. Error bars indicate 1 standard deviation, throughout.
Fig. 3
Fig. 3
NLS–dTomato protein delivery by actively or passively loaded vesicles. (a) Fluorescence of vesicles adsorbed to latex beads. 5×109 vesicles were adsorbed to latex beads for 2 h and analysed by flow cytometry. EVs, left; gesicles, right. (b) Comparison of nuclear-targeted (blue) and cytosolic (red) dTomato delivery by EVs (left) and gesicles (right). EVs from cells expressing no dTomato (grey) was a negative control. “Normalized fluorescence” is defined as the mean fluorescence of cells receiving vesicles divided by the mean fluorescence of cells receiving a medium change only. Duplicate wells of vesicle-receiving cells were analysed. Within each experiment, the number of vesicles added to each well was normalized (via vesicle counts determined NanoSight) such that each well received the same number of vesicles. *Significant difference was evaluated with a Student's t-test using a cut-off of p<0.05.
Fig. 4
Fig. 4
Comparative analysis of dTomato delivery by actively or passively loaded vesicles. For panels a–c, the cartoons at left summarize the experimental designs, and in the panels at right, each data point represents the average of duplicate wells of cells treated with the same type of vesicle. Error bars indicate 1 standard deviation. “Normalized fluorescence” is defined as the mean fluorescence of cells receiving vesicles divided by the mean fluorescence of cells receiving a medium change only. (a) Time course of EV delivery to cells. Grey squares: CD63–HA EVs; orange circles: CD63–MS2–HA EVs. The solid arrow represents cells that received a medium change after 4 h of EV treatment. The dashed arrow indicates that cells did not receive a medium change. Statistically significant differences (p<0.05, not shown for clarity): CD63–MS2–HA +/− medium change. An independent repeat of this experiment is shown in Supplementary Fig. 3a. (b) Time course of gesicle delivery to cells. Purple squares: VSVG–HA gesicles; green circles: VSVG–MS2–HA gesicles. Solid and dashed arrows carry the same meaning as in panel (a). Statistically significant differences (p < 0.05, not shown for clarity): VSVG–HA versus VSVG–MS2–HA at 4 and 16 h (comparisons were made for each time point), VSVG–HA +/− medium change, and VSVG–MS2–HA +/− medium change. An independent repeat of this experiment is shown in Supplementary Fig. 3c. (c) Comparison of delivery by gesicles from cells transfected with VSVG–HA (purple), VSVG–MS2–HA (green) or a 50:50 mix of VSVG–HA and VSVG–MS2–HA (hybrid gesicles, magenta). (d) dTomato RNA levels (normalized to GAPDH) in VSVG–HA gesicles (purple), VSVG–MS2–HA gesicles (green) or hybrid gesicles (magenta). Error bars indicate 1 standard deviation of technical duplicate samples. *Significant difference was evaluated with a Student's t-test using a cut-off of p < 0.05.
Fig. 5
Fig. 5
Fate of cargo delivered by HEK293FT-derived EVs and gesicles to PC-3 recipient cells. (a) Evaluating the role of translation in EV-mediated conferral of fluorescence to recipient cells. PC-3 cells were treated with vesicles for 4 h, and then fresh medium containing cycloheximide (CHX, gold square) or DMSO (black circle) was added. EVs (left); gesicles (right). An independent repeat of this experiment is shown in Supplementary Fig. 3e. (b) Proposed conceptual summary of the conclusions drawn for this model system: cargo molecule delivery via HEK293FT-derived vesicles to PC-3 recipient cells.

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