Cell fusion and some subcellular properties of heterokaryons and hybrids

The technique of somatic cell fusion has made it possible to study cell biology in an unusual and direct way. When cells are mixed in the presence of Sendai virus, their membranes coalesce, the cytoplasm becomes intermingled, and multinucleated homoand heterokaryons are formed by fusion of similar or different cells, respectively (1, 2). By fusing cells which contrast in some important biologic property, it becomes possible to ask questions about dominance of control processes, nucleocytoplasmic interactions, and complementation in somatic cell heterokaryons. The multinucleate cell may divide and give rise to mononuclear cells containing chromosomes from both parental cells and become established as a hybrid cell line able to propagate indefinitely in vitro. The chromosome constitution of the hybrid cells can then remain relatively stable, as in intraspecific crosses, or may be unstable, as in man-rode~nt interspecific hybrids which tend to lose human chromosomes selectively, as first noted by Weiss and Green (3). This combination of cell mating and chromosome segregation makes it possible to perform genetic analysis on somatic cells, to assign human gene products to individual chromosomes, and to study the control of gene expression in animal cells. Originally, cell hybridization was an uncontrolled, apparently spontaneous event which occurred rarely when different cells were simply mixed and co-cultivated. Barski et al. (4), and Ephrussi and Weiss (5) isolated and studied the first cell hybrids obtained by this method. The isolation of hybrid cells from such mixed cultures was greatly facilitated by the Szybalski et al. (6) and Littlefield (7, 8) adaptation of a selective medium containing hypoxanthine, aminopterin, and thymidine (HAT) to mammalian systems. Further progress followed the use of Sendai virus by Harris and Watkins to increase the frequency of heterokaryon formation (9). These authors exploited the observation by Okada that UV irradiation could be used to inactivate Sendai virus without loss of fusion efficiency (10). It was therefore possible to eliminate the problem of virus replication in fused cells. Since many cells carry receptors for Sendai virus, including those of different species, it became possible to fuse a variety of cell types, including differentiated and nondividing cells. The technique of cell fusion has already been applied to the study of many problems in the genetics and cellular biology of somatic cells. The purpose of this paper is to provide a selective review of experimental studies that are of special interest to the cell biologist, with emphasis on the principles and pitfalls underlying the use of this technique. Topics which have been extensively reviewed elsewhere will not be discussed in detail (11-14). These include linkage analysis and chromosome assignment (15), virus rescue by cell hybridization (12), and the application of cell hybridization to the study of malignancy (16).