Clonal zones were individually reseeded in 24-wells plate and screened for protein expression

GAPDH. All primer sequences are supplied in S1 Fluorescent Bead Uptake Experiments Normal or iPSC-derived keratinocytes were seeded onto gelatin-coated four-well chamber slides at a cell density of 2 104 cells/well in CnT-07 media. Cultures were allowed to expand until they were about 70% confluent. Cells were incubated with freshly sonicated fluorescent microspheres 0.5m in diameter . Microspheres were pre-incubated for 1 hour at 37C in CnT-07 media before being incubated with cells for different time points at a final concentration of 288,000 particles/ml in CnT-07 media. Media was removed and cells were washed 3 times in fresh cold media followed by a final wash in cold PBS to remove non-ingested microspheres. Cells were then fixed in ice-cold methanol for 10 minutes at RT, rinsed with PBS, coverslipped using Vectashield mounting media containing DAPI and examined using a Zeiss LSM 5 Exciter confocal laser scanning microscope. Quantitative analysis of the beads was performed by counting the number of internalized beads in 30 cells for each time point, randomly taken from 3 microscopic fields in 3 different experiments, and values are expressed as the mean value SEM. Statistical analysis was performed using the Student’s t test and significance level has been identified as p<0.05. In order to determine that only internalized beads were counted in the quantitative analysis we performed parallel phase contrast and fluorescence microscopy. Melanosome Isolation and Uptake Experiments The isolation of melanosomes for transfer was performed as previously described with a few modifications. Briefly, MNT-1 human melanoma cells were cultured in T75 tissue culture flasks. After reaching 95% confluence, the cells were harvested with trypsin-EDTA, washed with PBS, and stored in a PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19731037 freezer until used. After thawing the frozen MNT-1 cells, 1ml of cold lysis buffer pH 7.5, 1% Igepal CA-630, 0.01% SDS ) was added to the cells obtained from each T75 tissue culture flask and homogenized on ice using 20 strokes in 2 minutes with a Dounce glass/glass homogenizer. The homogenate was then stored at 4C for 20 minutes with mixing every 10 minutes. After centrifugation, the supernatants were transferred to new Eppendorf tubes and centrifuged again in the same manner. The supernatants were further centrifuged and the precipitates were washed twice by PBS with brief and gentle mixing in order to avoid dispersion and were centrifuged again. The pellets were then used as the melanosome-rich fraction. To prepare a dispersion of melanosomes, 100l of CnT-07 media was added to each melanosome pellet and was mixed by pipetting. When the 5 / 16 Pigmented Induced Pluripotent Stem Cell-Derived Skin Models melanosomal IMR 1 suspension was homogeneous, 50l was added to each normal or iPSC-derived keratinocyte containing gelatin-coated well of a 6-well plate for 24 hours. For experiments involving soybean trypsin inhibitor , an inhibitor of keratinocyte phagocytosis, 1mg/ml STI was added to the cells 2 hours before addition of the melanosomes and kept in the media for the duration of the 24 PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19728371 hour experiment. Keratinocytes adherent to the plate were washed 2 times with PBS to remove non-ingested melanosomes and were then processed for Fontana-Masson staining. Fontana-Masson Staining Normal or iPSC-derived melanocytes were washed twice with PBS and then fixed with ice-cold methanol for 10 minutes at RT. After washing the cells twice with dH2O, they were incubated with Fontana ammoni