A general method for heterokaryon detection using resonance energy transfer and a fluorescence-activated cell sorter

A simple and general labeling method has been adapted for the rapid detection and isolation of heterokaryons soon after somatic cell fusion. The method involves prelabeling each parental cell type by adding one of two hydrophobic fluorescent probes, F18 or R16, to the culture medium. These probes are nontoxic, nonmutagenic, and do not inhibit either cellular replication or the efficiency of fusion. Following polyethylene glycol (PEG)-induced fusion, heterokaryons are identified on a cell sorter as cells showing fluorescence as a result of nonradiative resonance energy transfer (RET) between the two fluorochromes. Optimal conditions are described for the unambiguous detection of heterokaryons by fusion-induced RET. The value of this method lies in the universal applicability of F18 and R16 as simple and direct membrane labels for any pair of parental cells, even those lacking selectable genetic markers or detectable antigenic differences. This potential for heterokaryon selection in any cell cross should significantly expand the range of cell types amenable to investigation through somatic cell genetics, while the rapidity of the method should facilitate the study of early events following fusion.