Quantifying the syncytialisation of human placental trophoblast BeWo cells grown in vitro

We have generated lines of BeWo cells that constitutively and stably express either histone H2B tagged with the green fluorescent protein (GFP), or the mitochondrial targeting sequence of subunit VIII of cytochrome c oxidase fused with a red fluorescent protein; one line has nuclei that fluoresce green, the other mitochondria that fluoresce red. Expression of these tagged proteins has no effect on the rates of DNA, RNA and protein synthesis, or on the amounts of human chorionic gonadotropin (hCG) secreted after treatment with forskolin. We used fluorescence-activated cell sorting (FACS) to monitor the extent of cell fusion (syncytialisation) between these two lines; fused cells are readily and accurately detected by their green/red fluorescence. This assay should prove useful in the investigation of the molecular mechanisms involved in trophoblast syncytialisation.