Behavior of differentiated hen nuclei in the cytoplasm of rat myoblasts and myotubules

Sir John Gurdon: father of nuclear reprogramming

Sir John Gurdon founded the field of nuclear reprogramming. His work set the stage for the ever burgeoning area of stem cell biology and regenerative medicine. Here I provide personal reflections on times I shared with John Gurdon and professional reflections of the impact of his ground-breaking research on my own development as a scientist and on the field in general. His paradigm-shifting experiments will continue to provoke scientists to think outside the box for many years to come.

Reprogramming mediated by cell fusion technology

This review is focused on recent advances in fusion-based reprogramming of cells of different pluripotent statuses or lineage origins. Recent findings are discussed from standpoints of both the developmental potency of hybrid cells generated by fusion of pluripotent embryonic stem (ES) cells, embryonal carcinoma (EC) cells, and somatic cells and epigenetic mechanisms and other aspects involved in the reprogramming process. Complete reprogramming occurs at least 5-7 days after fusion and includes at least two steps. (i) initiation at the heterokaryon stage and choice of the direction of reprogramming using an "all-or-none principle" to establish the dominance of one parental genome and (ii) "fixation" of the newly acquired expression profile by epigenetic mechanisms. The first step is realized without cell division, whereas the second requires cell proliferation. Reprogramming in hybrid cells is rapid and complete. Thus, cell fusion is a powerful tool for reprogramming.

Cytoplasmic activation of human nuclear genes in stable heterocaryons

We have induced the stable expression of muscle-specific genes in human nonmuscle cells. Normal diploid human amniocytes were fused with differentiated mouse muscle cells by using polyethylene glycol. The fusion product, a stable heterocaryon in which the parental cell nuclei remained distinct, did not undergo division and retained a full complement of chromosomes. This is in contrast with typical interspecific hybrids (syncaryons), in which the parental nuclei are combined and chromosomes are progressively lost during cell division. The human muscle proteins, myosin light chains 1 and 2, MB and MM creatine kinase and a functional mouse-human hybrid MM enzyme molecule were detected in the heterocaryons. Synthesis of these proteins was evident 24 hr after fusion and increased in a time-dependent manner thereafter. Our results indicate that differentiated mouse muscle nuclei can activate human muscle genes in the nuclei of a cell type in which they are not normally expressed, and that this activation occurs via the cytoplasm. The activators are still present in cells which have already initiated differentiation, are recognized by nuclei of another species, and do not diffuse between unfused cells. The reprogrammed amniocyte nuclei of stable heterocaryons provide a unique system in which to study the mechanisms regulating gene expression during cell specialization.

Reinitiation of DNA synthesis in postmitotic nuclei of myotubes by virus-mediated fusion with embryonic fibroblasts

Myotubes, whose nuclei have stopped DNA synthesis were fused with replicative embryonic fibroblasts. In heterokaryons the postmitotic muscle nuclei resumed DNA synthesis. Incorporation of radioactive thymidine into muscle, and also into fibroblast nuclei was dependent upon the time elapsed between virus-mediated fusion and administration of radioactive thymidine. Whereas incorporation into fibroblast nuclei diminished with time, there was an early increase of labelling into muscle nuclei followed by a decrease of incorporation of 3H thymidine. DNA synthesis was also dependent upon the ratio of noncycling (muscle) to cycling (fibroblast) nuclei. There was a greater incorporation of 3H thymidine into muscle and fibroblast nuclei in myotubes containing larger numbers of fibroblast nuclei. A model is discussed for the control of DNA synthesis in polykaryocytes derived from fusion of cycling and noncycling cells.

Thymidine kinase, DNA synthesis and cancer

A resume has been presented of some recent investigations which show that DNA synthesis can be initiated in many types of quiescent animal cells by external stimuli, by introducing a quiescent nucleus into the cytoplasm of a proliferating cell, or by a virus infection. The components of the DNA replication apparatus are described. It is shown that deoxyribonucleoside triphosphate pools increase substantially in animal cells at the time DNA synthesis is initiated due to the enhanced activities of enzymes functioning in nucleotide synthesis. Especially striking is the increase of thymidine kinase activity, indicating that this enzyme may be a useful marker of the shift from the quiescent to the replicative state. The thymidine kinase isozymes of vertebrate cells have been characterized. Thymidine kinase F, which is found principally in the cytosol, is the isozyme that increases when G1 (Go) phase cells are stimulated or infected with oncogenic viruses. Chick cytosol thymidine kinase F can also be reactivated by introducing differentiated chick erythrocyte nuclei into the cytoplasm of enzyme-deficient LM (TK-) mouse cells. Furthermore, herpesviruses code for distinctive, virus-specific thymidine kinase isozymes, so that another way to transform thymidine kinase-deficient LM TK-) cells to kinase-positive cells is by infecting them with UV-irradiated herpes simplex viruses. The experiments on the activation of DNA synthesis and thymidine kinase F activity have been discussed in the context of the proliferative activity in vivo and the immortalization in culture of neoplastic cells. These experiments suggest that genes determining cell cycle proteins are readily accessible to transcription and translation in essentially all nucleated cells. The tendency of transformed cells to become multinucleated after cytochaliasin B treatment also suggests that one important difference between malignant cells and most normal cells may be the ability of malignant cells to 'stockpile' the proteins (and/or their messenger RNAs) of the DNA replicative apparatus and to maintain the 'stockpiles' in progeny cells.

Heterokaryons in the analysis of genes and gene regulation

Cytological and chemical analysis of heterokaryons, the immediate product of cell fusion, offer new possibilities for studying the factors responsible for genetic regulation in eukaryotic cells. In comparison with proliferating cell hybrids the heterokaryon state offers the important advantage that a heterokaryon contains two complete genomes since chromosome loss does not occur, but since segregation and recombination are absent, heterokaryons cannot be used for gene mapping in the same way as proliferating cell hybrids. However, if two cell types carrying different genetic defects are fused the analysis can be used for studies of gene complementation. The biological information obtained with heterokaryons has emphasized the role of the cytoplasm in the control of nuclear activity. When a G1 nucleus is brought into contact with the cytoplasm of an S phase cell the G1 nucleus is stimulated to synthesize DNA. If the nucleus is brought into a mitotic cell, the chromatin of the G1 nucleus is forced to condense into prematurely condensed chromosomes. Inactive nuclei such as the dormant chick erythrocyte nucleus will be stimulated to initiate RNA and DNA synthesis when brought into contact with an active cytoplasm by cell fusion. Specific nuclear proteins have been shown to be responsible for this process of reactivation. Other inactive nuclei such as the nuclei of macrophages and spermatozoa have likewise been shown to be reactivated by fusion with active cells. The degree of activation in all of these cases appears to be determined by the state of the active cell. Inactive nuclei are activated to the same level as the active nucleus but seldom beyond this level. If differentiated cells are fused with undifferentiated cells, usually the differentiated character is lost rapidly after fusion. This observation is in agreement with several studies on proliferating cell hybrids indicating some type of negative control of differentiated properties. In heterokaryons obtained by fusion of cells of a similar type of histotypic differentiation usually coexpression of the differentiated markers is observed.

Detection of human and chick nuclear antigens in nuclei of chick erythrocytes during reactivation in heterokaryons with HeLa cells

Inactive nuclei of chick erythrocyte resume RNA synthesis and increase in volume and dry mass in heterokaryons made by virus-induced fusion of human tumor cells (HeLa) with chick erythrocytes. Nuclear growth is due primarily to migration of human macromolecules into the chick nucleus. Human nucleoplasmic antigens were detected in the nucleoplasm and human nucleolar antigens were detected in the nucleoli of reactivated chick erythrocyte nuclei. After some time, chick-specific nucleolar antigens appear in the nucleoli of both the reactivated chick nuclei and the HeLa cellnuclei. The results suggest that human nuclear proteins play an important part in the reactivation of the chick genome.