Cell culture and functional assays

3.3.1 Culture conditions

All cell lines were incubated in a water-saturated atmosphere with 5 % CO2 at 37°C. Rat basophilic leukemia (RBL) cells were cultured in RPMI 1640 medium supplemented with 10 % heat inactivated FCS and 100 µg/ml mixture of penicillin/streptomycin. For the selection of transfected cells 600 µg/ml geneticin was added. To thin out the cells they were washed with PBS and subsequently trypsinized with trypsin-EDTA for up to 25 minutes. All other cell lines (Ag8, peritoneal macrophages, hybridoma cells) were kept in RPMI 1640 medium without geneticin.

Human embryonic kidney cells (HEK293) cells were incubated in DMEM medium supplemented with the same ingredients as the RPMI 1640 medium. For cell selection a concentration of 1200 µg/ml geneticin was used. For maintenance cultures the concentration was lowered to 600 µg/ml. For the detachment of HEK293 cells the cells were washed gently with PBS and trypsinized with trypsin-EDTA for 2 minutes.

Centrifugation of all cell lines was done at 270 x g at rt for 5 minutes. All working steps which were relevant for cell culturing were done under sterile conditions with sterile solutions and sterile equipment.

3.3.2 Freezing and thawing of cells

For cell-storage cells were washed with PBS, trypsinized and centrifuged for 5 minutes at 270 x g at rt. The supernatant was decanted and the cell-pellet was gently resuspend in freezing-medium.

Afterwards the cell-suspension was transferred into pre-cooled 2 ml cryo-tubes and stored at -80°C.

For long term storage the cells were frozen in liquid nitrogen. Cell thawing was done in a water bath at 37°C. The cryo-tube was removed from the water bath after nearly the whole cell-suspension was molten and transferred into 10 ml of cell-culture medium. The cells were centrifuged at 270 x g for 5 minutes at rt and the supernatant was removed. The cell pellet was gently resuspended in cell culture medium and seeded in new culture dishes.

3.3.3 Cell counting

Adherent cells were washed with PBS, trypsinized and centrifuged at 270 x g for 5 minutes at rt.

Afterwards the cell pellet was resuspended in a suitable volume of cell culture medium. 10 µl of the cell-suspension was mixed with 90 µl of 0.5 % trypan blue-solution and pipetted onto a Neubauer counting chamber.

3.3.4 Cell transfection

3.3.4.1 Transfection of RBL-cells using electroporation

To express foreign DNA in a cell line it is necessary to clone the DNA into a suitable expression vector.

For our purpose we cloned the receptor DNA into the expression vector pEF1/Myc-HisA which contained a geneticin resistance cassette for the selection of positively transfected cells. The plasmid was linearised with the restriction enzyme ScaI (3.2.1.4) and extracted with phenol/chloroform (3.2.1.9 ). The cells were washed with PBS, trypsinized and 1 * 10⁷ cells were resuspended in 250 µl PBS. 18 µg of the linearised expression vector was pipetted into a pre-cooled electroporation cuvette. The cell suspension was added, gently mixed with DNA and incubated for 10 minutes on ice.

The electroporation was done at 960 µF and 260 V at rt. After the electroporation the cells were again cooled on ice for 10 minutes. Afterwards the cell suspension was transferred into 10 ml of culture medium and centrifuged at 270 x g for 5 minutes at rt. The supernatant was decanted and the cell pellet was resuspended in 1 ml of culture medium. The 1 ml cell suspension was added to

3 Materials and Methods

36 ml of culture medium and distributed in 0.5 ml aliquots on 24 well culture plates. To select the positively transfected cells the culture medium was exchanged for culture medium with 600 µg/ml geneticin. Visible colonies were transferred into 96 well plates and the expression level of receptors at the cell surface was checked using flow-cytometry (3.3.6.1).

3.3.4.2 Co-transfection of HEK293-cells using calcium-phosphate

HEK293 cells are susceptible for transfection by calcium-phosphate precipitates. The DNA is enclosed in the calcium-phosphate precipitates and is taken up by the cells over the cell membrane. The pEF1/Myc-HisA vector which contained the receptor-DmrA DNA was linearized using the restriction enzyme ScaI. β-Arrestin 2 –DmrC was cloned into pHet1 which was linearized with MluI 3.2.1.4). Both plasmids were purified using phenol/chloroform extraction (3.2.1.9).

One day before the transfection 6 * 10⁵ were seeded on multiple 10 cm culture dishes. 4 hours before the cells were transfected the medium was exchanged with DMEM medium from Invitrogen.

5 µg DNA from each construct was added to 50 µl 2.5 M CaCl2 and with ddH2O adjusted to a final volume of 500 µl. Afterwards 500 µl of 2x HBS-solution was added dropwise with additional snapping of the tube. Immediately afterwards the mixture was added to the cells. 24 hours post transfection the medium was removed and 900 µl of pre-warmed 10 % glycerol (v/v) was added to the cells and removed shortly afterwards. For selection purposes 1200 µg/ml genetecin was added to the medium. Effect of cell selection was visible after 14 days. Further treatment of positive cell clones was identical to RBL-cells.

The calcium-phosphate transfection method could also be used for transient transfections. In contrast to the stable transfection the transient transfection of cells is not permanent and the transfected plasmids must not be linearized. For the transient transfection 3 * 10⁵ cells were seeded in a well of a 6 well plate one day before transfection. For the transfection 3.3 µg DNA was added to 16.6 µl 2.5 M CaCl2 and adjusted with ddH2O to a final volume of 166 µl. Afterwards 166 µl 2x HBS-solution was added dropwise. The remainder of the transfection protocol was identical to the stable transfection.

3.3.4.3 Subcloning of positively selected cell-clones

To ensure that the positively selected cell clones did not contain any contaminants the cells were subcloned. Cells were adjusted to 1.3 * 10⁴ cells per ml and were diluted threefold (260/62/20 cells per ml) in a total medium volume of 25 ml. One drop of this cell-suspension was added to 100 µl medium per well of a 96 well plate resulting in a minimal cellular concentration of 0.8 cells per well.

Separately grown colonies were analyzed using flow-cytometry (3.3.6.1).

3.3.5 Functional assays

3.3.5.1 Ca²⁺-mobilization assay

To measure the cytosolic Ca²⁺-shift after cellular stimulation the membrane permeable dye Indo-1-AM was used. In the presence of Ca²⁺ the emission spectrum changed (475 to 400 nm), which was used to detect the intracellular calcium mobilization. Cells were adjusted to 1.5 * 10⁶ and resuspended in 700 µl RPMI 1640 medium with 5 % FCS. 1 µM Indo-1-AM mixed with pluronic-acid F127 (0.015 % v/v) was carefully added to the cell suspension (25’/30°C/350 rpm) in darkened tubes.

700 µl of RPMI 1640 medium with 10 % FCS was added (10’/30°C/350 rpm). Cells were washed and resuspended in Krebs-Ringer-solution. The 400 nm/475 nm ratio was recorded for 25 seconds by flow cytometry and resulting data were normalized.

3.3.5.2 N-acetyl-β-D-glucosaminidase (NAGA) release

To test the functional integrity of receptors expressed by RBL cells the ligand-induced and G protein-mediated NAGA release from transfected cells was measured. RBL cells are myeloid cells which contain NAGA within prestored vesicles.

RBL cells were adjusted to 4 * 10⁶ cells in 1 ml HAG-CM buffer containing 0.25 % BSA and 5 µl 100 % cytochalasin (w/v) and were incubated for 20 minutes at 37°C. In parallel, 10 µl of 1 % triton-X (v/v) was added to 90 µl of the cell suspension and incubated for 4 minutes at 37°C to obtain maximal cell lysis (positive control). The sample was centrifuged (3200 x g/ 5’ at 4°C) and the cell-free supernatant was used for calibration purposes with samples diluted 1:2 ratio in HAG-CM buffer. The ligand (0.5 - 250 nM) in 10 µl HAG-CM buffer was added to 90 µl of cell-suspension and incubated at 37°C for 4 minutes and centrifuged (3200 x g/5’ at 4°C). Supernatant (25 µl) of the triton lysate and of the ligand-stimulated cells were incubated (2 hours/37°C) in a microtiter plate with 125 µl of 4 Nitrophenyl N-acetyl-β-D-glucosaminide. The reaction was stopped by adding 50 µl of 1 M glycerol-buffer and the optical density at 450 nm was determined in a plate photometer. The standard curve was used to calculate the NAGA release (% of total cellular NAGA content).

3.3.5.3 Phosphorylation/activation of ERK1/2 MAP-kinases after ligand and AP21967/AP20187 stimulation

6 * 10⁵ HEK293-cells were seeded onto lysine-coated 6 well plates and cultivated overnight in FCS-free DMEM medium containing 0.02 % BSA. The next day cells were incubated (60’/37°C) in fresh DMEM medium without serum. After cellular stimulation the plates were chilled on ice and were detached with treatment buffer and lysed (5’/95°C/sonication). The lysates were used for immunoblotting (3.5.2) with anti-phospho-ERK1/2 detection antibodies. As a loading control the immunoblot was reprobed using anti-pan-ERK1/2-antibody.

A similar protocol was followed to probe a diverse range of signaling pathways using human phospho-MAPK array kit from R&D system. Cells were prepared as described and the kit was used following the manufacturer’s guidelines.

3.3.5.4 Quantification and visualization of internalization- and recycling processes of CXCR4- and CCR5-receptors

In the presence or absence of receptor agonists and antagonists the expression of CXCR4- and CCR5-receptors on the cell surface is a subject of constant changes. These changes can be either detected by biotin-streptavidin (3.6) or direct receptor staining. If necessary, the cells were biotinylated (3.6) and adjusted to 6 * 10⁶ cells. The cells were resuspended in 2 ml BM medium and stimulated with receptor-agonist/-antagonist or AP21967/AP20187 (30’/37°C). To measure the recycling rate of the receptor the samples were acid washed to remove the ligand and resuspended in ligand-free BM medium (30’/37°C). Through the whole process samples were taken, washed with EM-, BM-medium and FACS-buffer and transferred into wells of a 96 well plate (4°C). Quantification was performed by flow cytometry (3.3.6.1). The percentage of expressed receptors on the cell surface was calculated on the base of unstimulated control cells. For the visualization of internalized receptors immunfluorescence microscopy (3.3.6.2) was done. The procedure was identical to the quantification with the exception that all steps were done with adherent cells.

3 Materials and Methods

3.3.6 Fluorescence-based assays 3.3.6.1 Flow-cytometry

To detect the amount of CXCR4 and CCR5 on the cell surface the cells were stained with fluorophor-labeled antibodies or streptavidin. Cells were adjusted to a defined cell density of cells per ml. The staining was done in a 96-well plate (60’/4°C/dark). In between the cells were washed with FACS-buffer. The samples were resuspended in FACS-buffer and transferred into tubes. For the calibration of the flow cytometer non labeled cells were used. Routinely, 10,000 cells were measured per sample.

3.3.6.2 Immunfluorescence miscroscopy

Prior to the staining 1 * 10⁵ cells were seeded on sterile round cover slips, which were placed in 24 well plates (overnight at 37°C). If necessary the cells were biotinylated (3.6) and stimulated with the corresponding ligand or AP21967/AP20187. Cells were fixed with paraformaldehyd (PFA) (20’/37°C) and quenched with a 50 mM NH4Cl solution (30’/37°C). To permeabilize the cell membrane, the cells were incubated with a gelatin-saponin solution (15’/37°C). For the staining the primary antibody was added (60’/on ice) followed by the detection antibody or streptavidin (60’/on ice/dark). Between all steps the samples were washed either with sterile PBS or gelatin-saponin solution. The cover slips were sealed with mounting medium and placed on a microscope slide and subsequently dried (overnight/rt/dark). Cells were analyzed by confocal laser microscopy using a 63x1.3 objective and the oil immersion technique.