CD63eGFP tagged HEK293T Extracellular Vesicle Properties.
To generate fluorescently labeled EVs for analyzing the kinetics and uptake of extracellular vesicles, HEK293T cells were transfected with a plasmid carrying CD63-eGFP fusion protein. CD63 is a tetraspanin protein commonly enriched in the membrane of exosomes making it an optimal target for EV fluorescent tagging (34,35). Spent media was collected from HEK293T cell culture and EVs were isolated as previously reported (8). We compared the size and distribution of EVs isolated from CD63-eGFP transfected HEK293T cells to non-transfected HEK293T cells. CD63-eGFP transfected HEK293T EVs displayed an average median diameter of 110.28 nm and 103.616 nm respectively, as measured by nanotracking software (Fig. 1A), which is consistent with the reported size of HEK293T EVs. No significant differences in median diameter (p = 0.1615), distribution (p = 0.4225), or concentration (data not shown) of EVs isolated from non-transfected and CD63-eGFP transfected HEK293T cells were observed. eGFP labeling did not alter size of HEK293T EVs (Fig. 1B).
IFC assay was conducted to determine if the CD63-eGFP was associated with EVs. As a fluorescent negative control, 1.34 µM beads in buffer solution (Fig. 1C top), lacked fluorescence when exposed to the 488 nm excitation wavelength, but were visible in brightfield (BF) and side scatter (SSC). Untagged HEK293T EVs were negative in the BF, GFP, and SSC, suggesting a small size below the BF threshold and lack of fluorescence (Fig. 1D, middle). The absence in BF signifies an EV size smaller than 300 nm, which also suggests minimal swarming of EVs. Lastly, CD63-eGFP tagged EVs are negative in BF and positive in the GFP channel signifying positive fluorescence of the HEK293T EVs (Fig. 1C, bottom). Collectively, these results show that the isolated HEK293T EVs have standard size and protein marker profiles consistent with previous reports of HEK293T exosomes and eGFP labeling does not alter the size of HEK293T EVs (5,21).
Using a commercially available flow cytometry-based method to measure common EV markers, we determined the overall EV tetraspanin profile (5). Isolated HEK293T EVs from control and CD63-eGFP expressing HEK293T cells were positive for standard EV markers including CD9, CD63, and CD81 as measured in relative fluorescence units (Fig. 1D). As previously reported, CD29 was also found on the surface of HEK293T EVs and CD63-eGFP transfected HEK293T EVs (5). These results indicate that the isolation and tagging methods for HEK293T EV result in EVs with common HEK293T exosome markers.
2. Active uptake of EK293T EVs.
Two inhibitory internalization assays were performed. EVs were co-cultured with recipient cells at 4 °C (cold) or fixed with paraformaldehyde prior to co-culture with HEK293T EVs. The treatments decreased the presence of eGFP-labelled EVs in the recipient cells compared to recipient cells co-cultured with EVs under physiological conditions (Fig. 2A). Cold and fixed inhibitory assays reduced the spot count (Cold: p = 0.0127, Fixed: p = 0.0078), intensity (Cold: p = 0.0105, Fixed: p = 0.0374), and maximum pixel (Cold: p = 0.0159, Fixed: p = 0.0149) of fluorescence signals in recipient cells without treatments, indicating inhibition of EV uptake. These results conversely infer that eGFP localization and increases in output parameters signify that HEK293T EVs are internalized for the following uptake assays.
3. Dose dependent HEK293T EV uptake
To develop a standard dose curve for the IFC platform, HEK293T EVs were co-cultured with HEK293T recipient cells at increasing doses ranging from 0 to 20,000 EVs per cell at 37°C. Representative IFC images exhibited a visual increase of eGFP fluorescence with elevated doses of EVs (Fig. 3A). The lowest number of EVs that could be detected was 6,000 EVs per co-cultured HEK293T cell. At this level, spot count (p = 0.0012), intensity (p = 0.0075), and maximum pixel (p = 0.0005), measurements were significantly greater than recipient cells without EVs (Fig. 3B-D). Therefore, doses of 6,000 HEK293T EVs is the low threshold for uptake in our experimental condition. Similarly, doses of 10,000 and 20,000 EVs had higher spot count (10,000: p = 0.0009; 20,000: p < 0.0001), intensity (10,000: p < 0.0001; 20,000: p < 0.0001), and maximum pixel (10,000: p < 0.0001; 20,000: p < 0.0001) compared to cells without EVs. Comparing between the higher doses, there are no significant differences in spot count (6,000 vs. 10,000: p = 0.999, 10,000 vs. 20,000: p = 0.0927), intensity (6,000 vs. 10,000: p = 0.8482, 10,000 vs. 20,000: p = 0.999), and maximum pixel count (6,000 vs. 10,000: p = 0.6056, 10,000 vs. 20,000: p = 0.5281) between 6,000 and 10,000, along with 10,000 vs 20,000. Similarly, comparing between 6,000 and 20,000, there is no statistical difference in spot count (p = 0.0787) and intensity (p = 0.8083). There is a significant difference in maximum pixel between 6,000 and 20,000 (p = 0.0140). Overall, the yield curve displays a significant dose dependence in all parameters (Spot, Intensity, Max Pixel, p < 0.0001). These results indicate that HEK293T EV uptake is dose dependent with a minimum threshold of 6,000 HEK293T EVs per cell.
4. HEK293T EV temporal uptake
Using 6,000 EVs per cell, HEK293T EVs were co-cultured with HEK293T cells for increasing lengths of time prior to IFC, ranging from 5 minutes to 24 hours. Length of EV exposure played a key role in the amount of visible fluorescence in the recipient cells, declining after 12 hours (Fig. 4A). Initially, 30 minutes of co-culture displayed a significant increase in spot count (p = 0.0081) suggesting a possible trend towards EV uptake, but not in other uptake parameters (Intensity: p = 0.3073, Max Pixel: p = 0.0952) (Fig. 4B-D). At two hours of co-culture, significantly higher spot count (p = 0.0028), intensity (p = 0.0420), and maximum pixel (p = 0.0006) were recorded compared to the recipient cells without EVs. Again, at 4 hours of co-culture all parameters were greater than controls (Spot count: p = 0.0003, Intensity: p < 0.0001, Max pixel: p < 0.0001). Intensity and maximum pixel continued to be higher than controls at 4, 12, and 24 hours of co-culture. There were no differences in any uptake parameters between 4 hours and 12 hours of co-culture (Spot: p = 0.999, Intensity: p = 0.5797 Maximum Pixel: p = 0.2489). However, intensity (p = 0.0191), and maximum pixel (p = 0. 0027) decreased between 12 and 24 hours of co-culture (Fig. 4C-D). Similar to the dose curve, HEK293T EV uptake is time dependent with consistent EV uptake at 4 hours of incubation and a peak at 12 hours. Collectively, a dose of 6,000 EVs per cell seeded and a co-culture of 4 hours has been standardized for the following uptake assays.
5. Comparative uptake of HEK293T EVs by Multiple Cell Lines
The hypothesis that EV uptake is a selective process where EVs are preferentially taken up by cells of their own origin was tested using IFC. HEK293T EVs were co-cultured with HEK293T cells or other cell lines: epithelial (C3A liver cells), endothelial (Human Umbilical Vein Endothelial Cells), and neural (SH-SY5Y Glioblastoma Cells.). eGFP fluorescence is more abundant in HEK293T cells as compared to the other cell types (Fig. 5A). Compared to C3A and HUVECs, HEK293T cells had significantly higher fluorescence intensity (C3A: p = 0.0321; HUVEC: p = 0.0055) (Fig. 5C), when co-cultured with HEK293T EVs. Additionally, HEK293T cells had higher maximum pixel (C3A: p = 0.0221; HUVEC p = 0.0079; SH-SY5Y: p = 0.0486) (Fig. 5D) as compared to all other recipient cell lines (Fig. 5B). Regarding intensity, SH-SY5Y cells was significantly higher than HUVECs when co-cultured with HEK293T EVs (p = 0.0304). These results support HEK293T EV selective uptake up by HEK293T cells compared to other cell lines in vitro.
6. Differentiation status of neural cells and HEK293T EV internalization
Since EVs have been implicated for therapeutic and delivery purposes targeting neural diseases, human neural stem cells (hNSCs) and mature human neurons were used as recipient cell lines in our system to examine if the differentiation status of the recipient cell plays a role in selective uptake of EVs. Representative images from IFC displayed visual evidence of uptake in both cell types, but with the greatest eGFP localization in hNSCs (Fig. 6A). hNSCs co-cultured with HEK293T EVs have higher spot count (p = 0.0082) and max pixel (p = 0.0083) as compared to mature neurons. Together, these results suggest that differentiation status of neural cells affects uptake of HEK293T EVs.