Evidence for enzyme subunit complementation in somatic cell heterokaryons

Somatic cell heterokaryons derived from normal human fibroblasts which had different glucose-6-phosphate dehydrogenase (G6PD) electrophoretic variants, types A and B, were examined for their G6PD pattern. A hybrid band of activity with intermediate migration, in addition to the A and B bands, was observed in such heterokaryons. These results directly demonstrate that enzyme subunit complementation can take place in somatic cell heterokaryons, and suggest that this technique may be important for elucidating the molecular basis of the genetic heterogeneity seen with many human single enzyme defects.     

Uncoupling of muscle-specific protein expression in myocyte X myoblast heterokaryons

The inducibility of several rat skeletal muscle proteins was examined in heterokaryons formed by fusing differentiated chick myocytes to undifferentiated rat myoblasts. Chicken and rat proteins were distinguished using species-specific antibodies or by their different migrations in polyacrylamide or agarose gels. Both rat skeletal myosin light chain 1 and rat α-tropomyosin were induced in the heterokaryons. In contrast, neither rat acetylcholine receptors nor creatine kinase could be detected. These results suggest that chick myocytes may contain quantities of regulatory factors that are sufficient for the activation of some but not all of these rat muscle-specific proteins within the cellular context of the heterokaryon.     

Extinction of differentiated glial functions in somatic cell hybrids is incomplete

Rat glioma cells of clone C₆ were hybridized in vitro with mouse L cells of clone A9 or with freshly isolated mouse macrophages, and the hybrids were assayed for glial cell functions. C₆ cells expressed high levels of 2′,3′-cyclic nucleotide 3′-phosphohydrolase (CNP; EC 3.1.4.37), β-hydroxybutyrate dehydrogenase (HBDH; EC 1.1.1.30), glycerol-3-phosphate dehydrogenase (GPDH; EC 1.1.1.8), and inducibility of GPDH by hydrocortisone (HC). A9 cells and macrophages had very low activities of these functions. Hybrids between C₆ and A9 or between C₆ and macrophages had greatly reduced activities of these functions, but the hybrids expressed significantly higher activities than the non-glial parent. This incomplete extinction was not due to fusion of two glioma cells with one L cell or macrophage. The difference in GPDH activity in the hybrids as compared with the non-glial parent was due to incomplete shut-off of GPDH of the glial parent, and not to an increase in GPDH production by the non-glial genome.     

Isolation of somatic hybrids by cloning Nicotiana heterokaryons in nurse cultures

Fusion of Nicotiana knightiana Goodsp. and kanamycin resistant Nicotiana sylvestris Speg. et Com. protoplasts was induced by polyethylene glycol treatment. Heterokaryons were isolated by micropipette and transferred to nurse cultures of albino cells. Colonies originating from the heterokaryons could subsequently be distinguished by their green colour. The somatic hybrid nature of four such colonies was confirmed by isoenzyme pattern, kanamycin resistance and restored morphogenic potential. An additional kanamycin resistant line with characteristic Nicotiana knightiana isoenzymes was also found indicating that the drug resistance in the kanamycin resistant parent is under cytoplasmic control.Abbreviations ADH alcohol dehydrogenase - Nk Nicotiana knightiana     

Extinction of alpha-fetoprotein gene expression in somatic cell hybrids involves cis-acting DNA elements.

alpha-Fetoprotein (AFP), a liver-specific protein, is extinguished in somatic cell hybrids formed by the fusion of mouse hepatoma cells (BWTG3) with rat fibroblast cells (JF1). Our studies show that the extinction of mouse AFP expression in these somatic cell hybrids may involve at least two cis-acting regulatory domains, i.e., the enhancer elements and a tissue-specific promoter region, which are located in the 5'-flanking region of the AFP gene.     

Dopa oxidase expression in fibroblast x melanoma fusions. Extinction in heterokaryons and maintenance in non-dividing cybrids

Pigmented B-16 mouse melanoma cells were fused with chick embryo fibroblasts or fibroblast cytoplasts and maintained as heterokaryons or non-dividing cybrids, respectively. These single cells were examined ultrastructurally for evidence or pigment gene expression using a cytochemical test for dopa oxidase, the initial enzyme in the conversion of dopa to melanin. Heterokaryons showed significantly less enzyme activity than control cells, whereas non-dividing cybrids showed no significant difference. Therefore, the presence of the intact nuclear membranes in the heterokaryons did not serve as a barrier to the interactions resulting in extinction of differentiated function(s). However, the presence of the fibroblast nucleus was necessary to elicit continued response.     

Extinction of differentiated glial functions in somatic ce-l hybrids is incomplete.

Rat glioma cells of clone C₆ were hybridized in vitro with mouse L cells of clone A9 or with freshly isolated mouse macrophages, and the hybrids were assayed for glial cell functions. C₆ cells expressed high levels of 2′,3′-cyclic nucleotide 3′-phosphohydrolase (CNP; EC 3.1.4.37), β-hydroxybutyrate dehydrogenase (HBDH; EC 1.1.1.30), glycerol-3-phosphate dehydrogenase (GPDH; EC 1.1.1.8), and inducibility of GPDH by hydrocortisone (HC). A9 cells and macrophages had very low activities of these functions. Hybrids between C₆ and A9 or between C₆ and macrophages had greatly reduced activities of these functions, but the hybrids expressed significantly higher activities than the non-glial parent. This incomplete extinction was not due to fusion of two glioma cells with one L cell or macrophage. The difference in GPDH activity in the hybrids as compared with the non-glial parent was due to incomplete shut-off of GPDH of the glial parent, and not to an increase in GPDH production by the non-glial genome.     

Plasticity of cell fate: insights from heterokaryons.

Experiments with somatic cell hybrids and stable heterokaryons have demonstrated that differentiated cells exhibit a remarkable capacity to change. Heterokaryons have been particularly useful in determining the extent to which the differentiated state of a cell is plastic. Cell fate can be altered by a change in the balance of positive and negative trans-acting regulators. Although a single regulator may be sufficient in certain environments to trigger a change in cell fate, that regulator may be ineffective in other cell contexts where it encounters a different composition of regulators.     

Expression of differentiated properties in fetal liver cells and their somatic cell hybrids

A line of murine fetal liver cells is described and characterized. Hybrids between these differentiated cells and a non-differentiated fibroblast line were isolated and studied for the expression of the differentiated characteristics of the parent liver line. These include expression and inducibility for the enzyme tryptophan pyrrolase, ability to accumulate glycogen granules, and a characteristic morphology and growth rate, all of which were suppressed in the hybrid. The results obtained from sequential chromosome counts of the hybrid indicate that the modal number drops from about 100 to about 72 in 5 months, a fact that may be related to the extreme differences in generation times of the two parents.     

Changes in the properties of the resident L virus in somatic cell hybrid lines

Production of the N-tropic L virus by hybrids of A9 cells (subline of mouse L cells) was found to be influenced by the genotype of the partner cells inasmuch as it was suppressed by B-type cells. Five hybrid lines with the B-type partner were subjected to prolonged in vitro passage during which all lost chromosomes, accompanied in some by phenotypic changes. These five hybrid cell lines resumed virus production detectable either by antigen induction on JLS-V9 cells, and/or by focus formation on mouse embryo fibroblasts. By infection of both N- and B-type embryo cells, the host range of the reappearing viruses was found to be different from the L virus; it was B-tropic in one and NB-tropic in two cases. The results indicate that rearrangements and loss of genetic material in the hybrid cells may influence the infective properties of their resident C-type viruses.     

Stability of inactive X-chromosome in mouse embryoid body-mule cell and transformed mouse cell-mule cell heterokaryons

Cloned fibroblast cells of female mule derivation, expressing only the horse G6PD phenotype, were fused with established mouse cells, with spontaneously and virally transformed mouse cells, and with embryoid bodies from a transplantable mouse teratoma. Heterokaryons were harvested at various intervals after fusion, and tested for their G6PD activity patterns by electrophoresis on Cellogel sheets. No expression of the donkey G6PD phenotype in these heterokaryons could be detected, although hybrid G6PD bands formed between mouse and horse subunits were clearly visible. These observations imply that neither the cytoplasm of the embryoid bodies, nor of the DNA and RNA tumour virus-transformed mouse cells can induce depression of the G6PD locus on an inactive X-chromosome and provide further evidence for the genetic stability of the inactive X-chromosome in mammalian somatic cells.