Correction of a genetic defect in a mammalian cell

Ganoderma Fusions with High Yield of Ergothioneine and Comparative Analysis of Its Genomics

Ergothioneine (EGT), an exceptional antioxidant found ubiquitously across diverse living organisms, plays a pivotal role in various vital physiological regulatory functions. Its principal natural sources are mushrooms and animal liver tissues. Ganoderma spp., a traditional Chinese food and medicinal mushroom, boasts high concentrations of EGT. To advance the development of novel Ganoderma spp. strains with enhanced EGT yields, we employed an efficient Ganoderma spp. protoplasmic fusion system. Through molecular and biological characterization, we successfully generated seven novel fusion strains. Notably, fusion strain RS7 demonstrated a remarkable increase in mycelial EGT production (12.70 ± 1.85 mg/L), surpassing the parental strains FQ16 and FQ23 by 34.23% and 39.10%, respectively. Furthermore, in the context of the fruiting body, fusion strain RS11 displayed a notable 53.58% enhancement in EGT production (11.24 ± 1.96 mg/L) compared to its parental strains. Genomic analysis of the RS7, the strain with the highest levels of mycelial EGT production, revealed mutations in the gene EVM0005141 associated with EGT metabolism. These mutations led to a reduction in non-productive shunts, subsequently redirecting more substrate towards the EGT synthesis pathway. This redirection significantly boosted EGT production in the RS7 strain. The insights gained from this study provide valuable guidance for the commercial-scale production of EGT and the selective breeding of Ganoderma spp. strains.

Fusion between human and Drosophila melanogaster cells induced by polyethylene glycol

Polyethylene glycol was used to induce interspecific somatic cell fusion between human fibroblasts (stock F6) and Drosophila melanogaster cells from established cell lines (Cl 82 and 11 P102), characterized by different ploidy levels. The present investigation defines some parameters for Drosophila cell fusion and interspecific fusion between Drosophila and human cells. The cytological analysis provided evidence of spontaneous as well as induced human-Drosophila heterokaryon formation. The presence in the same cell of two types of nuclei, distinguishable because of their different size and morphology, was confirmed autoradiographically by 3H-thymidine pre-labelling of Drosophila cells. Furthermore, the retained DNA synthetic activity and some examples of mitotic figures of both types of nuclei in the heterokaryons indicate the viability of the fused cells.

Chinese hamster cells with a minichromosome containing the centromere region of human chromosome 1

We describe a series of primary and secondary hamster-human hybrids which have selectively retained a small amount of human DNA. The hybrid XJM12.1.3 contains an estimated 4000-8000 kb of human DNA, and for a secondary hybrid derived from it, XEW8.2.3, our estimate is 1000-2000 kb. The hybridization of Southern blots of DNA from these hybrids with a variety of human satellite DNA probes reveals that these lines include centromere sequences of human chromosome 1. The identifiable human DNA is in the form of a minichromosome, as detected by in situ hybridization in the light microscope and in the electron microscope. At mitosis, the minichromosome can be observed to have kinetochores and to be associated with microtubules. Therefore, it can segregate in a stable fashion. It may be significant that in the selection of the hybrids we had selected for a human gene which has been mapped on human chromosome 1.

Insertion of muntjac gene segment into hamster cells by cell fusion

Indian muntjac diploid cells that have only three pairs of easily discernible large chromosomes were fused with hamster cells deficient in hypoxanthine guanine phosphoribosyltransferase (HGPRT) using polyethylene glycol. Cells that survived in hypoxanthine-aminopterine-thymidine (HAT)-oubaine medium were analyzed. Hybrid cells containing both muntjac and hamster chromosomes in a given cell were not found. Instead, the cells had the same chromosomal sets as those of either parental muntjac or hamster cells. A clonal isolate that had the same chromosomal sets as those of parental hamster cells were analyzed in detail and showed the following characteristics: (1) portions of the survival curve in various concentrations of HAT medium were intermediate between those of parental cells; (2) expressions of both muntjac and hamster antigen(s) were detected by immunofluorescence staining; (3) the mobility of the enzyme HGPRT in gel electrophoresis differed from that of parental hamster or muntjac cells. These results indicate that the clonal isolate (AD202h) is a somatic cell hybrid of hamster and muntjac that contains chromosomal sets of hamster with an inserted segment of the muntjac genome, including HGPRT. The formation of such an unusual hybrid and a possible explanation of transfer of some gene segments in the hybrid cell in this system are discussed.

Restoration of metabolic cooperation in heterokaryons between HGPRT-deficient mouse A9 fibroblasts and chick embryo erythrocytes

Genetic determinants of metabolic cooperation were studied by fusing chick erythrocytes to HGPRT- mammalian cells. Heterokaryons were then tested for their ability to incorporate [3H]hypoxanthine and to transfer radioactive material to HGPRT- recipient cells. Chick erythrocytes (CE) have nuclei which are inactive but contain the HGPRT gene and some cytoplasmic HGPRT enzyme activity. They are unable, however, to cooperate with HGPRT- cells. Of the two mammalian cell lines used, the human GM29 line is HGPRT- and capable of functioning as a receptor cell in cooperation experiments with HGPRT+ cells. The HGPRT- mouse A9 line on the other hand is unable to cooperate. Immediately after fusion, both types of heterokaryons incorporated [3H]hypoxanthine, indicating the presence of some chick HGPRT enzyme contributed by the erythrocyte partner at the time of fusion. While the CE-GM29 heterokaryons participated in metabolic cooperation shortly after fusion, the CE-A9 heterokaryons did not. However, four days after fusion, i.e., at a time when the erythrocyte nucleus had been reactivated, the CE-A9 heterokaryons did cooperate. This suggests that in CE-A9 heterokaryons the genes required for metabolic cooperation are expressed by the previously dormant chick erythrocyte nucleus.

Marsupial--mouse cell hybrids containing fragments of the marsupial X chromosome

Hybrids were obtained from fusions of HPRT-deficient mouse fibroblasts and marsupial lymphocytes. These hybrids retained no identifiable marsupial chromosomes, but all expressed the marsupial form of HPRT. Half the clones also expressed marsupial PGK-A, and half of these also marsupial G6PD; no other marsupial allozyme markers were detected. Since G6PD is known to be sex linked in these species, we conclude that Hpt and Pgk-A are also located on the X chromosome and the markers lie in the order Hpt-Pgk-A-Gpd.

Asynchronous DNA replication and asymmetrical chromosome loss in Chinese hamster-mouse somatic cell hybrids

A number of parameters were measured in a series of 12 hybrid cell clones from Chinese hamster and mouse cells to test the hypothesis that asymmetrical chromosome loss may result from asynchrony in the replication of the two parental sets of chromosomes. All clones tended to lose telocentric (mouse) chromosomes with culture time, irrespective of the starting ratio of parental chromosomes, and in all clones, biarmed (hamster) chromosomes appeared to complete DNA replication slightly earlier than telocentrics. However, no quantitative correlation could be established between the degree of asynchrony in chromosome DNA replication and the extent of chromosome loss. It appeared that telocentric chromosomes were lost more readily from those clones which started with a high hamster-to-mouse chromosome ratio.

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.

Transfer of the human gene for hypoxanthine-guanine phosphoribosyltransferase via isolated human metaphase chromosomes into mouse L-cells

We have transferred the human gene for hypoxanthine-guanine phosphoribosyltransferase (HPRT, EC 2.4.2.8; IMP:pyrophosphate phosphoribosyltransferease) via isolated metaphase chromosomes from human HeLa S3 cells into murine A9 cells which lack functional murine HPRT activity, using the technique of McBride and Ozer (Proc, Nat. Acad. Sci. USA 70, 1258-1262, 1973). Three transformed clones were isolated which contained human HPRT activity as determined by electrophoretic and immunochemical assays. Twenty human isozymes other than HPRT whose genes have been assigned to 14 human chromosomes were found to be absent in our transformed clones. Moreover, the human isozymes of hlucose-6-phosphate dehydrogenase (EC 1.1.1.49; D-glucose 6-phosphate:NADP 1-oxidoreductase) and phosphoglycerate kinase (EC 2.7.2.3;ATP:3-phospho-D-glycerate 1-phosphotransferase), whose genes have been linked with the HPRT gene to the long are of the human X chromosome, were also absent. On the basis of the known linkage relationships of the three markers, we thereby suggest that the transferred piece of human genetic material is smaller than 20% of the human X chromosome or less than 1% of the human genome. This estimate assumes a normal syntenic relationship for the long arm of the X chromosome in HeLa S3 cells. In agreement with this conclusion, no human chromosomes could be detected in our transformed clones. When grown under nonselective conditions about 3% of the gene transfer cells lost the human HPRT marker per cell generation. Transformants that had lost human HPRT activity were subjected to hypoxanthine-aminopterin-thymidine selection. The frequency of revertants to the HPRT(+) phenotype was less than 1 x 10(-6), and two revertants that were obtained possessed the mouse electrophoretic phenotype. These results argue against a stable integration of the human donor genetic material into the mouse recipient genome.

Incorporation of isolated chromosomes and induction of hypoxanthine phosphoribosyltransferase in Chinese hamster cells

Evidence is presented for the uptake of radioactive-labeled isolated Chinese hamster chromosomes following incubation with Chinese hamster cells. Metaphases were found which contained radioactive labeled chromosomes in a very low frequency, and in some of the labeled chromosomes only one chromatid was labeled. Incubation of hypoxanthine phosphoribosyltransferas (HPRT)-deficient Chinese hamster cells with chromosomes isolated from HPRT+ Chinese hamster or human cells resulted in the appearance of HPRT+ cells. Clones derived from these cells were isolated in HAT medium. Cells in mitosis during incubation with the chromosomes yielded thr-e times more HPRT+ clones than did cells in interphase. The intraspecies combination involving recipient cells and chromosomes from Chinese hamster origin yielded significantly higher numbers of HPRT+ clones than did the interspecies system using human chromsomes and Chinese hamster recipient cells (5 X 10(-5) and 6 X 10(-6) respectively). Electrophoresis of HPRT from Chinese hamster cells treated with human chromosomes revealed the pattern of the human enzyme.

Oncogenesis by interspecific interaction of malignant murine and non-malignant hamster cells in vitro

A clone of Cloudman S91 murine melanoma was fused in vitro with non-malignant hamster cheek pouch cells by means of lysolecithin, and the putative hybrid progeny cells, HCP-MM, were found to be highly malignant in hamster, but not in appropriate mice. A malignant clone of HCP-MM cells was shown to have hamster species-specific surface antigens (as demonstrated by immunofluorescence and the cytotoxic antibody) and hamster-like lactate dehydrogenase and NAD-dependent malate dehydrogenase isoenzyme profiles. Nevertheless, chromosomes similar to those of both murine and hamster parental cells could be distinguished in cells of this malignant clone and in hamster tumor grafts by the method of trypsin-Giemsa banding. A majority of the murine chromosomes, however, appeared to be lost. This study indicates that a murine melanoma previously found untransplantable in hamsters could produce a highly malignant and lethal tumor for hamsters after being mixed in vitro with non-malignant hamster cells, in the presence of a fusing chemical. It is not as yet certain whether the production of transformed cells in vitro and of highly malignant tumors in the hamster (both with predominantly hamster properties) required heterosynkarion formation between the murine melanoma and hamster cheek pouch cells. Nevertheless, our results suggest that the presence of the murine melanoma, and possibly the interaction of its genome with non-malignant hamster cells, was implicated in this process.

Acquisition of chick cytosol thymidine kinase activity by thymidine kinase-deficient mouse fibroblast cells after fusion with chick erythrocytes

Chick-mouse heterokaryons were obtained by UV-Sendai virus-induced fusion of chick erythrocytes with thymidine (dT) kinase-deficient mouse fibroblast [LM(TK(-))] cells. Autoradiographic studies demonstrated that 1 day after fusion, [(3)H]dT was incorporated into both red blood cell and LM(TK(-)) nuclei of 23% of the heterokaryons. Self-fused LM(TK(-)) cells failed to incorporate [(3)H]dT into nuclear DNA. 15 clonal lines of chick-mouse somatic cell hybrids [LM(TK(-))/CRB] were isolated from the heterokaryons by cultivating them in selective hypoxanthine-aminopterin-thymidine-glycine medium. LM(TK(-)) and chick erythrocytes exhibited little, if any, cytosol dT kinase activity. In contrast, all 15 LM(TK(-))/CRB lines contained levels of cytosol dT kinase activity comparable to that found in chick embryo cells. Disk polyacrylamide gel electrophoresis and isoelectric focusing analyses demonstrated that the LM(TK(-))/CRB cells contained chick cytosol, but not mouse cytosol dT kinase. The LM(TK(-))/CRB cells also contained mouse mitochondrial, but not chick mitochondrial dT kinase. Hence, the clonal lines were somatic cell hybrids and not LM(TK(-)) cell revertants. The experiments demonstrate that chick erythrocyte cytosol dT kinase can be activated in heterokaryons and in hybrid cells, most likely as a result of functions supplied by mouse fibroblast cells.

Cytoplasmic inheritance of chloramphenicol resistance in mouse tissue culture cells

A chloramphenicol-resistant mutant, isolated from mouse A9 cells, was enucleated and fused with a nucleated chloramphenicol-sensitive mouse cell line. Resultant fusion products, cytoplasmic hybrids (or "cybrids"), were selected as resistant to chloramphenicol, and had the nuclear markers and chromosome complement of the chloramphenicol-sensitive parent. These cybrids appeared at the high frequency of 2-8 per 10(4) cells plated. Neither parent produced any colonies when plated under identical selective conditions. Fusion between enucleated chloramphenicol-sensitive cell fragments and the chloramphenicol-sensitive cell produced no resistant colonies, suggesting that chloramphenicol resistance is not due to an increase in the ratio of cytoplasm to nucleus. Furthermore, fusions between resistant and sensitive nucleated cells produced resistant hybrids at a frequency 100 times less than that of resistant cybrids. Thus, these stable chloramphenicol-resistant cybrids result from the fusion of a chloramphenicol-resistant cytoplasm with a chloramphenicol-sensitive cell. It is proposed, therefore, that chloramphenicol resistance is a cytoplasmically inherited characteristic in this mouse cell line.

Modulation of the activity of an avian gene transferred into a mammalian cell by cell fusion

Mouse A9 cells, deficient in hypoxanthine phosphoribosyltransferase (EC 2.4.2.8), were fused with normal chick erythrocytes and selected in hypoxanthine-aminopterin-thymidine medium for cells with hypoxanthine phosphoribosyltransferase activity. Recovered hybrid cells produced the chick hypoxanthine phosphoribosyltransferase exclusively, as demonstrated by electrophoretic mobility and immunoprecipitation tests, even though no chick chromosomes or chick cell-surface antigens could be identified in the hybrids. Surprisingly, the expression of the chick hypoxanthine phosphoribosyl-transferase activity in the mouse/chick hybrids required the presence of aminopterin in the growth medium; in its absence, enzyme synthesis decreased markedly. Because of the rapid and reversible modulation of hypoxanthine phosphoribosyltransferase activity, the hybrid cells could proliferate equally well in media containing hypoxanthine-aminopterin-thymidine or 8-azaguanine. Cellular selection was definitely ruled out as a possible cause. These results confirm previous reports that specific genetic information can be selectively transferred from one cell to another of a distant species. Furthermore, they demonstrate that an avian gene, whose activity is normally expressed constitutively, can become facultative when integrated into a mammalian cell. This seems to be the first instance where heterologous gene activity has been shown to be reversibly modulated in hybrid cells.

Identification of chick chromosomes in cell hybrids formed between chick erythrocytes and adenine-requiring mutants of Chinese hamster cells

When chick erythrocytes were fused with adenine-requiring mutants of Chinese hamster cells of two different complementation classes and grown in adenine-free medium for 1 week or longer, mitotic cells were observed containing both chick and Chinese hamster metaphase chromosomes within a single cell. Stable hybrid clones were subsequently isolated that possessed, in addition to a complete Chinese hamster genome, a single specific chick chromosome. Hybrids resulting from fusions involving different adenine mutants retained different, identifiable chick chromosomes. Thus, the two chick genes for the endogenous synthesis of adenine were assigned to the respective chick chromosomes.

Restoration of hypoxanthine phosphoribosyl transferase activity in mouse 1R cells after fusion with chick-embryo fibroblasts

Fusion of the 1R mouse cell, which lacks activity of hypoxanthine phosphoribosyl transferase (EC 2.4.2.8), with chick-embryo fibroblasts yielded progeny cells that survived in hypoxanthine-aminopterin-thymidine selective medium. This property and the failure of the progeny to survive in 8-azaguanine indicated that hypoxanthine phosphoribosyl transferase activity was present. Electrophoretic analysis revealed that the enzyme was of mouse, not chick, origin. These observations are consistent with the operation of a regulator gene responsible for the absence of hypoxanthine phosphoribosyl-transferase activity in the 1R cell and its presence in the progeny.

Transfer of genetic information by purified metaphase chromosomes

Transfer of genetic information from isolated mammalian chromosomes to recipient cells has been demonstrated. Metaphase chromosomes isolated from Chinese hamster fibroblasts were incubated with mouse A(9) cells containing a mutation at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus. Cells were plated in a selective medium, resulting in death of all unaltered parental A(9) cells. However, colonies of cells containing hypoxanthine phosphoribosyl transferase (EC 2.4.2.8) appeared with a variable frequency of about 10(-6) to 10(-7). The enzyme from these cells was indistinguishable from that from Chinese hamster cells, as shown by DEAE-cellulose chromatography and gel electrophoresis, and differed clearly from the mouse enzyme. The colonies, thus, did not result from reversion of A(9) parental cells to wild type, but appeared to represent progeny of individual cells that had ingested chromosomes, replicated, and expressed the hprt gene. These colonies differed from each other in stability of expression of the transferred gene.