Extracellular vesicle techniques

2.2.2.1 Isolation of microvesicles (MV) and exosomes (Exo) 2.2.2.1.1 Cell culture supernatants

To isolate EV from adherent cells, the latter were cultured in seven T175 cell culture flasks up to 60-80% confluence, then washed twice with PBS and incubated for 48 h in culture medium supplemented with 10% particle-free FCS. Human Mϕ were seeded in 10 cm cell culture Petri dishes (Sarstedt) in a concentration of 3,5 ∙ 10⁶ cells per dish and incubated for 24 h in RPMI +1% particle-free FCS. The particle-free FCS was generated by pelleting endogenous EV by ultracentrifugation over night at 100.000 g and 4°C.

Cell culture supernatants were collected and centrifuged at 750 g for 5 min, followed by 1.500 g for 15 min to pellet cells and debris. MV were pelleted by ultracentrifugation in a Sw 32 Ti rotor at 14.000 g and 4°C for 35 min. The pellet was washed once in PBS and then

resuspended in PBS or RIPA lysis buffer. For the isolation of Exo, the supernatant of the 14.000 g centrifugation step was filtered (0,22 µm filter, Sarstedt) and ultracentrifuged at 100.000 g and 4°C for 2 h. The Exo pellet was washed once in PBS in a TLA-120.2 rotor and then resuspended in PBS or RIPA lysis buffer. MV and Exo counts were routinely determined as described in 2.2.3.2.

Fig. 6: Schematic representation of the EV isolation protocol

EV were isolated from cell culture supernatants by a series of differential (ultra)centrifugation steps as indicated.

The particle-free supernatant (sn) after precipitation of Exo was employed as negative control for the experiments.

2.2.2.1.2 Platelet concentrates

P-MV were isolated according to the protocol in 2.2.2.1.1 from outdated platelet concentrates (outdated <2 days) which were provided by the Department of Transfusion Medicine, University Medical Center Göttingen.

2.2.2.1.3 Peripheral blood from cancer patients and controls

For the isolation of total MV and Exo from the peripheral blood of cancer patients, blood samples were collected from cancer patients with metastatic disease who did not receive chemotherapy at the time of sample acquisition. This setting should guarantee the maximal count of tumor-derived EV in the blood, while preventing the presence of apoptotic bodies or activated EV from dying cells which might be present during administration of chemotherapy.

As negative control, blood was also collected from matched tumor-free patients or healthy individuals. Samples were collected through a 19 gauge butterfly needle and dispended into tubes containing either EDTA (1,6 mg / ml blood, Sarstedt) or lithium-heparin (16 I.E. / ml blood, Sarstedt). Serum was obtained by centrifugation at 1.200 g for 15 min and separation from the blood cells further supported through the application of a serum filter (Seraplas^®, Sarstedt). The serum was centrifuged at 1.500 g for 15 min to pellet debris and then stored at -20°C or directly subjected to EV isolation as described in 2.2.2.1.1. The isolation of tumor-derived EV from cancer patients by magnetic-activated cell sorting (MACS) is further described in 2.2.4.3.

2.2.2.2 Sucrose gradient ultracentrifugation

For sucrose gradient preparations, T-MVS and T-ExoS were isolated according to chapter 2.2.2.1.1 and pellets were resuspended in 500 µl 0,25 M sucrose. The vesicles were then applied onto a sucrose step gradient which was prepared by layering decreasing sucrose density solutions (2 ml each) upon one another as listed in Tab. 9. The gradient was centrifuged in a Sw 32.1 Ti rotor for 16 h at 100.000g and 4°C.

Tab. 9: Sucrose step gradient

2,5 M sucrose [µl] HEPES buffer [µl] molarity [M] density [g ml^-1]

500 4500 0,25 1,03

1500 3500 0,75 1,06

2000 3000 1,00 1,09

2500 2500 1,25 1,16

3000 2000 1,50 1,19

3500 1500 1,75 1,22

4000 1000 2,00 1,25

4500 500 2,25 1,28

Eight fractions with 2 ml each were collected and proteins were precipitated by adding 18 ml ice-cold acetone and incubating overnight at -20°C. Samples were centrifuged for 30 min at 10.000 g and 4°C. The supernatant was aspirated and protein pellets washed in 4 ml ddH2O + 16 ml acetone (-20°C) until the remaining sucrose had been washed out. Pellets were dried for 1 h at room temperature and the protein was resuspended in 80 µl RIPA lysis buffer for Western Blot analysis.

HEPES buffer (20 mM):

 4,766 g HEPES

 Ad 1 l ddH2O and adjust pH at 7,4

2,5 M sucrose solution:

 42,75 g sucrose

 Ad 50 ml 20mM HEPES buffer

To solve the sucrose completely, the solution was carefully heated and stirred for at least 1 h at room temperature.

2.2.2.3 Labeling of MV through PKH26 staining

To visualize uptake of MV into recipient cells, MV were labeled with PKH26 (Sigma).

PKH26 is a red-fluorescent dye which intercalates non-specifically into lipid membranes and can be used for live cell tracking. Up to 100 µg MV were incubated with 250 µl working solution for 5 min under constant mixing by pipetting up and down. To stop the staining, 250 µl particle-free FCS were added and incubated for 1 min. The suspension was diluted with 500 µl RPMI-1640 and MV were pelleted at 14.000 g and 4°C for 35 min. The MV pellet was washed once in PBS and then resuspended in PBS for further experiments.

PKH26 working solution:

 250 µl Diluent C

 1 µl PKH26

2.2.2.4 Coupling of rhWnt5a to EV

In order to investigate if rhWnt5a can also associate unspecifically with EV, MV or Exo (100 µg each) from the Wnt5a-negative cell line MCF-7 were incubated with 100 ng/ml rhWnt5a for 12 h at 37°C. The reaction was carried out in 640 µl particle-free MCF-7 supernatant in case that additional soluble factors in the tumor cell supernatant are required for Wnt5a binding to the vesicles. Subsequently, MV or Exo were isolated from the supernatant according to the protocol in 2.2.2.1.1, washed once in PBS and resuspended in RIPA lysis buffer for Western Blotting. As negative control, MV and Exo were incubated in particle-free MCF-7 supernatant without addition of rhWnt5a and treated as described above.

To assess whether and how much of the added rhWnt5a remained unbound in the supernatant after 12 h of incubation, the supernatant after precipitation of MV or Exo was additionally loaded on Western Blot gels.