Cell cycle dependence of the reactivation of chick erythrocyte nuclei after transplantation into mouse L929 cell cytoplasts

Cell fusion and plasticity

Cell plasticity is a central issue in stem cell biology. In many recent discussions, observation of cell fusion has been seen as a confounding factor which calls into question published results concerning cell plasticity of, particularly, adult stem cells. An examination of the voluminous literature of "somatic cell fusion" suggests the relatively frequent occurrence of "spontaneous" cell fusion and shows that the complicated cellular phenotypes which it can give rise to have long been recognized. Here, a brief overview of this field is presented, with emphasis on studies of special relevance to current work on cell plasticity.

Reactivation of DNA replication in nuclei from terminally differentiated cells: nuclear membrane permeabilization is required for initiation in Xenopus egg extract

We have used Xenopus egg extract to investigate the requirements for reactivation of DNA replication in nuclei isolated from terminally differentiated chicken erythrocytes. Previous work has shown that reactivation of erythrocyte nuclei in egg extract is accompanied by chromatin decondensation, nuclear envelope reformation, and the accumulation of egg lamin, LIII. However, in those studies, erythrocyte nuclei were prepared by methods that were not designed to maintain the selective permeability of the nuclear membrane, and as such, it is not clear if loss of nuclear membrane integrity played a role in the reactivation process. Therefore, the purpose of this study was to determine if changes in nuclear membrane permeability are required for reactivation of erythrocyte nuclei in egg extract. Nuclei with intact nuclear membranes were prepared from erythrocytes with streptolysin O and permeable nuclei by treatment of intact nuclei with the detergent Nonidet-P40. Like permeable nuclei, most intact nuclei decondensed, imported nuclear protein, and accumulated lamin LIII from the extract. However, unlike permeable nuclei, which replicated extensively in the extract, few intact nuclei initiated replication under the same conditions. These data demonstrate that permeabilization of the nuclear membrane is required for reactivation of DNA replication in terminally differentiated erythrocyte nuclei by egg extract and suggest that loss of nuclear membrane integrity may be a general requirement for replication of quiescent cell nuclei by this system.

The nuclear membrane determines the timing of DNA replication in Xenopus egg extracts

We have exploited a property of chicken erythrocyte nuclei to analyze the regulation of DNA replication in a cell-free system from Xenopus eggs. Many individual demembranated nuclei added to the extract often became enclosed within a common nuclear membrane. Nuclei within such a "multinuclear aggregate" lacked individual membranes but shared the perimeter membrane of the aggregate. Individual nuclei that were excluded from the aggregates initiated DNA synthesis at different times over a 10-12-h period, as judged by incorporation of biotinylated dUTP into discrete replication foci at early times, followed by uniformly intense incorporation at later times. Replication forks were clustered in spots, rings, and horseshoe-shaped structures similar to those described in cultured cells. In contrast to the asynchronous replication seen between individual nuclei, replication within multinuclear aggregates was synchronous. There was a uniform distribution and similar fluorescent intensity of the replication foci throughout all the nuclei enclosed within the same membrane. However, different multinuclear aggregates replicated out of synchrony with each other indicating that each membrane-bound aggregate acts as an individual unit of replication. These data indicate that the nuclear membrane defines the unit of DNA replication and determines the timing of DNA synthesis in egg extract resulting in highly coordinated triggering of DNA replication on the DNA it encloses.

Transient correction of genetic defects in cultured animal cells by introduction of functional proteins

Material was introduced into cultures of cells by using the method of scrape loading, in which cells are simply rubbed from the surface of a plastic tissue culture dish by a rubber-tipped rod in the presence of a macromolecule of interest. The volume of solution introduced into cells was comparable to that generally injected in the direct microinjection method with glass capillaries, that is, about 50 to 100 fl per cell. Genetic defects (lack of hypoxanthine-guanine phosphoribosyltransferase and thymidine kinase) in several cell lines were transiently corrected by scraping the cells in the presence of crude cell extracts prepared from wild-type cells.

Transient correction of genetic defects in cultured animal cells by introduction of functional proteins

Material was introduced into cultures of cells by using the method of scrape loading, in which cells are simply rubbed from the surface of a plastic tissue culture dish by a rubber-tipped rod in the presence of a macromolecule of interest. The volume of solution introduced into cells was comparable to that generally injected in the direct microinjection method with glass capillaries, that is, about 50 to 100 fl per cell. Genetic defects (lack of hypoxanthine-guanine phosphoribosyltransferase and thymidine kinase) in several cell lines were transiently corrected by scraping the cells in the presence of crude cell extracts prepared from wild-type cells.

Autoregulation of tubulin synthesis in enucleated cells

The effects on tubulin messenger RNA levels and tubulin protein synthesis of treating cells with microtubule-depolymerizing drugs or directly microinjecting cells with tubulin have suggested that non-polymerized tubulin depresses its own synthesis. The precise level of this control is unclear. It has been shown that enucleated cells, termed cytoplasts, retain many properties of the original cell, including maintenance of cell shape, pinocytic activity and locomotion as well as biosynthetic activities such as protein synthesis and replication of cytoplasmic viruses. Furthermore, cytoplasts retain most of the components of the cytoskeleton including the centrioles. If cytoplasmic activities alone are responsible for regulating tubulin biosynthesis, cytoplasts should contain the necessary components. To distinguish between regulation which would occur in the nucleus, that is, alterations in mRNA synthesis or modifications of the mRNA, from alterations in mRNA stability and/or translatability which would take place in the cytoplasm, we examined the autoregulation of tubulin synthesis in enucleated cells. Here, we report that enucleated mouse fibroblasts retain the ability to turn off tubulin protein synthesis in response to microtubule depolymerization, the reduction in tubulin synthesis being accompanied by a corresponding decrease in tubulin mRNA levels. Thus, transcription, processing and transport of tubulin mRNA from the nucleus are not likely to be the loci of regulation. Instead, tubulin must reduce, either directly or indirectly, the translatability of its own mRNA.

Alterations in chromatin conformation are accompanied by reorganization of nonchromatin domains that contain U-snRNP protein p28 and nuclear protein p107

The intranuclear distribution of nuclear matrix-associated protein p107 and the 28-kD Sm antigen of U-snRNPs have been studied using double-label immunofluorescence and immunoperoxidase electron microscopy. In interphase nuclei of HeLa cells, Novikoff hepatoma cells, and rat kangaroo kidney cells, p107 was confined to discrete interchromatin domains. The domains had an irregular contour, with an average diameter of 1-1.5 micron. Each domain appeared to be composed of interconnected granules. The Sm antigen colocalized and appeared concentrated in these domains but also showed some general nucleoplasmic distribution. During mitosis, the interchromatin domains disassembled such that the Sm portion redistributed to the perichromosomal and spindle regions and the p107 component redistributed throughout the mitotic cytoplasm. During anaphase, p107 assembled into discrete clusters throughout the mitotic cytoplasm. The Sm antigen was not a component of these clusters. Double-label immunofluorescence with anti-p107 and the anti-DNA tight-binding protein, AhNa1, showed that the extranuclear p107 domains assumed an interchromatin localization only after the chromosomes had decondensed. The correlation between chromosome decondensation and the occurrence of p107 within interchromatin domains was also observed during chicken erythrocyte nuclear reactivation. We propose that the discrete interchromatin domains that contain p107 and p28 may be important for processing and splicing of RNA and that their structural assembly within nuclei is sensitive to the presence of the transcriptionally active conformation of chromatin.

Intracellular determinants of cell aging

Several important findings have been made since our observation that normal human and animal cells have a finite capacity to replicate and function. Among these are that: an inverse relationship exists between donor age and population doubling potential; more than 100 functional increments and decrements occur in cultured normal human cells before they age and die; normal tissue transplanted seriatim in vivo reveals a finite replicative and functional capacity; a direct relationship may exist between species maximum lifespan and population doubling potential of their cultured fibroblasts; the latent period increases as a function of age; cells from patients with accelerated aging syndromes undergo fewer population doublings than do age matched controls; and cultured fibroblasts from longer lived species have greater DNA repair capacity than do cells from species with shorter lifespans. Efforts to determine the location of the intracellular chronometer that controls these events has shown that the chronometer is intranuclear. The phenomenon of senescence has been found to be dominant over immortality.

Activation of dormant genes in specialized cells

In several experimental systems the genomic capacity in specialized cells can be assessed by examining the activation of dormant genes. Since some of these specialized cells can be induced to change cell phenotype, all cell specializations do not necessarily involve irreversible genetic changes.

Programmed macromolecular synthesis in regenerating karyoplasts

Conditions for the preparation, purification, and maintenance of karyoplasts which could regenerate to reform whole viable cells were defined. Results of biochemical analyses of such karyoplasts at various times during regeneration indicated that a reproducible biosynthetic program was followed. Thus, an examination of the polypeptides made during regeneration by two-dimensional gel electrophoresis showed that the pattern of radiolabeled polypeptides synthesized at each time studied was specific and was significantly different from that observed at other times during regeneration. Polypeptides associated with three major cellular fractions--nuclear, cytoskeletal-microtrabecular, and soluble--were among the most dramatically regulated molecules. Other polypeptides, such as the major components of microfilaments and intermediate filaments, were synthesized at relatively constant rates and were assembled into structures throughout regeneration. Likewise, microtubules appeared to be reformed throughout regeneration, even in the absence of identifiable centriole-associated organizing centers. Finally, analysis of DNA synthesis by autoradiography showed that, even when prepared from whole cells synchronized at the G1/S interface, karyoplasts could not begin making DNA until they had regenerated an almost complete complement of cytoplasm.

Programmed macromolecular synthesis in regenerating karyoplasts

Conditions for the preparation, purification, and maintenance of karyoplasts which could regenerate to reform whole viable cells were defined. Results of biochemical analyses of such karyoplasts at various times during regeneration indicated that a reproducible biosynthetic program was followed. Thus, an examination of the polypeptides made during regeneration by two-dimensional gel electrophoresis showed that the pattern of radiolabeled polypeptides synthesized at each time studied was specific and was significantly different from that observed at other times during regeneration. Polypeptides associated with three major cellular fractions--nuclear, cytoskeletal-microtrabecular, and soluble--were among the most dramatically regulated molecules. Other polypeptides, such as the major components of microfilaments and intermediate filaments, were synthesized at relatively constant rates and were assembled into structures throughout regeneration. Likewise, microtubules appeared to be reformed throughout regeneration, even in the absence of identifiable centriole-associated organizing centers. Finally, analysis of DNA synthesis by autoradiography showed that, even when prepared from whole cells synchronized at the G1/S interface, karyoplasts could not begin making DNA until they had regenerated an almost complete complement of cytoplasm.

Identification of key regulated events early in the life of hybrid animal cells constructed by nuclear transplantation

Reconstituted cells were constructed by fusion of cytoplasts from the human diploid fibroblast cell strain Detroit 532 and karyoplasts from the mouse fibroblast cell line A9. Several cellular properties were examined during the first 48 hr after nuclear transplantation. (i) The overall morphology of the cells originally resembled that of the cytoplasmic donor, Detroit 532, but rapidly changed to approximate that of the nuclear donor, A9. However, definitive changes in the microfilament structure of the reconstituted cells were not seen until 24-48 hr after fusion. These observations support the idea that the presence or absence of an ordered array of microfilament bundles is not the sole determinant of cell shape. (ii) Although cytoplasts and karyoplasts were prepared from cultures of randomly growing cells, the first division of reconstituted cells occurred in a synchronous manner. However, the initiation of DNA synthesis was not synchronized. It thus appeared that, in their first cell cycle, the cells had a G2 period of variable length. The results further suggest that the cytoplasm of interphase fibroblasts contains the material necessary to initiate or support DNA synthesis in a transplanted nucleus but not entry into mitosis. (iii) A two-dimensional gel electrophoretic analysis of polypeptide synthesis in reconstituted cell cultures showed that synthesis directed by transplanted mouse nuclei could be detected as early as 3-6 hr after fusion. Some of the mouse polypeptides detected at the earliest time points studied were not among the major polypeptides synthesized by the parental A9 cells. By about 48 hr after fusion, the pattern of polypeptides produced by reconstituted cells was almost indistinguishable from that of the nuclear donor parent cells.

Status of the nuclear matrix in mature and embryonic chick erythrocyte nuclei

The adult chicken erythrocyte nucleus was found to lack an internal nuclear matrix: even milder extraction procedures resulted in the production of empty shells of pore complex-lamina together with loose aggregates of core histone. In contrast, rat liver nuclei showed a typical intranuclear salt-resistant skeleton. These results show that an internal matrix is not an obligatory nuclear component, and is not required for the spatial organization of chromatin. 5-day-old embryonic erythrocytes did, however, contain an interchromatinic nuclear matrix, suggesting a correlation between the presence of matrix structures, and nuclear 'activity'.

Generation of an internal matrix in mature avian erythrocyte nuclei during reactivation in cytoplasts

When fused with mouse L-cell cytoplasts, chick erythrocyte nuclei enlarge, take up proteins from the host cytoplasm, and recommence RNA synthesis. We found that during this transition the erythrocyte nuclei gain an internal nuclear matrix, thus providing a novel approach to questions concerning the nature of the salt-resistant intranuclear skeleton. A new method for preparation and examination of the nuclear matrix in situ is also described.

Cytoplasmic modification of nuclear gene expression

A review is presented on 1) the autonomous nature of mammalian cell cytoplasm and 2) the cytoplasmic modification of nuclear gene expression. Topics include a discussion of cytoplasmic suppression of tumorigenicity. It is proposed that alterations in DNA methylation patterns may be a possible mechanism to explain cytoplasmic modification of nuclear gene expression.

Globin synthesis in hybrid cells constructed by transplantation of dormant avian erythrocyte nuclei into enucleated fibroblasts

The polypeptides synthesized by mature embryonic erythrocytes prepared from the peripheral blood of 14- to 15-day-old chicken embryos were analyzed by two-dimensional gel electrophoresis. Fewer than 200 species of polypeptides were detected; the major polypeptides made at this time were identified as the alpha A-, alpha D-, and beta-globin chains. The dormant erythrocyte nuclei were next reactivated to transcriptional competence by transplantation into enucleated mouse or chicken embryo fibroblasts, with frequencies of cytoplast renucleation of about 50 and 90%, respectively. Since large numbers of hybrid cells could be constructed, a biochemical analysis was possible. Electrophoretic analysis of the [35S]methionine-labeled polypeptides made in the hybrid cell types showed that polypeptides having the mobilities of only two (alpha A and alpha D) of the three major adult globin chains were made as major constituents of the hybrid cells. However, analysis of 14C-amino acid-labeled polypeptides revealed that a beta-like polypeptide that lacked methionine was also synthesized in large amounts. This polypeptide was tentatively identified as the early embryonic globin species rho. Globin synthesis was detected as early as 3 h after nuclear transplantation and as late as 18 h, the last time measured in these experiments. It appeared that globin polypeptides made at very early times were translated at least partially from chicken messenger ribonucleic acid introduced into the hybrid cells during fusion, whereas those made at later times were translated primarily from newly synthesized globin messenger ribonucleic acid. The potential usefulness of this hybrid cell system in analyzing mechanisms regulating globin gene expression is discussed.

Cytoplasts can transfer factor(s) that stimulate quiescent fibroblasts to enter S phase

Cytoplasts prepared from fibroblasts arrested by hydroxyurea were fused with serum-arrested, quiescent fibroblasts. In contrast to unfused mono-nucleated cells in the same culture, a sizable fraction of these cybridoids entered S phase in the absence of extracellular serum stimulation. Because DNA synthesis commenced only after a considerable lag following fusion, it was concluded that the cytoplasts contain factors which initiate the progression of quiescent cells toward S phase.

Reactivation of DNA synthesis in aging diploid human skin fibroblasts by fusion with mouse L karyoplasts cytoplasts and whole L cells

Diploid human skin fibroblasts derived from an 82-year-old donor with a 21-28 cell population doubling (CPD) range (where 28 CPD marked the end of the in vitro life span of the cells) were fused with whole L cells, L karyoplasts and L cytoplasts. The proportion of human nuclei incorporating tritiated thymidine after fusion was measured autoradiographically. Statistically significant increases in the labeling indices were found in the human nuclei in hybrid, heterodikaryon and cybrid cells when compared to control unfused human cells. Fusion of human diploid fibroblasts with human cytoplast derived from cells of the same CPD showed no significant changes in the labeling indices of the human nuclei.

Globin synthesis in hybrid cells constructed by transplantation of dormant avian erythrocyte nuclei into enucleated fibroblasts

The polypeptides synthesized by mature embryonic erythrocytes prepared from the peripheral blood of 14- to 15-day-old chicken embryos were analyzed by two-dimensional gel electrophoresis. Fewer than 200 species of polypeptides were detected; the major polypeptides made at this time were identified as the alpha A-, alpha D-, and beta-globin chains. The dormant erythrocyte nuclei were next reactivated to transcriptional competence by transplantation into enucleated mouse or chicken embryo fibroblasts, with frequencies of cytoplast renucleation of about 50 and 90%, respectively. Since large numbers of hybrid cells could be constructed, a biochemical analysis was possible. Electrophoretic analysis of the [35S]methionine-labeled polypeptides made in the hybrid cell types showed that polypeptides having the mobilities of only two (alpha A and alpha D) of the three major adult globin chains were made as major constituents of the hybrid cells. However, analysis of 14C-amino acid-labeled polypeptides revealed that a beta-like polypeptide that lacked methionine was also synthesized in large amounts. This polypeptide was tentatively identified as the early embryonic globin species rho. Globin synthesis was detected as early as 3 h after nuclear transplantation and as late as 18 h, the last time measured in these experiments. It appeared that globin polypeptides made at very early times were translated at least partially from chicken messenger ribonucleic acid introduced into the hybrid cells during fusion, whereas those made at later times were translated primarily from newly synthesized globin messenger ribonucleic acid. The potential usefulness of this hybrid cell system in analyzing mechanisms regulating globin gene expression is discussed.

Heterogeneous binding of high mobility group chromosomal proteins to nuclei

A dramatic difference is observed in the intracellular distribution of the high mobility group (HMG) proteins when chicken embryo fibroblasts are fractionated into nucleus and cytoplasm by either mass enucleation of cytochalasin-B-treated cells or by differential centrifugation of mechanically disrupted cells. Nuclei (karyoplasts) obtained by cytochalasin B treatment of cells contain more than 90 percent of the HMG 1, while enucleated cytoplasts contain the remainder. A similar distribution between karyoplasts and cytoplasts is observed for the H1 histones and the nucleosomal core histones as anticipated. The presence of these proteins, in low amounts, in the cytoplast preparation can be accounted for by the small percentage of unenucleated cells present. In contrast, the nuclei isolated from mechanically disrupted cells contain only 30-40 percent of the total HMGs 1 and 2, the remainder being recovered in the cytosol fraction. No histone is observed in the cytosol fraction. Unike the higher molecular weight HMGs, most of the HMGs 14 and 17 sediment with the nuclei after cell lysis by mechanical disruption. The distribution of HMGs is unaffected by incubating cells with cytochalasin B and mechanically fractionating rather than enucleating them. Therefore, the dramatic difference in HMG 1 distribution observed using the two fractionation techniques cannot be explained by a cytochalasin-B-induced redistribution. On reextraction and sedimentation of isolated nuclei obtained by mechanical cell disruption, only 8 percent of the HMG 1 is released to the supernate. Thus, the majority of the HMG 1 originally isolated with these nuclei, representing 35 percent of the total HMG 1, is stably bound, as is all the HMGs 14 and 17. The remaining 65 percent of the HMGs 1 and 2 is unstably bound and leaks to the cytosol fraction under the conditions of mechanical disruption. It is suggested that the unstably bound HMGs form a protein pool capable of equilibrating between cytoplasm and stably bound HMGs.

Construction of viable mouse-human hybrid cells by nuclear transplantation

Viable interspecies cytoplasmic-nuclear hybrid cells were constructed by fusion of karyoplasts prepared from the highly tumorigenic A9 mouse fibroblast cell line and cytoplasts prepared from the Detroit 532 normal human diploid cell strain. The identity of the hybrid cells was ascertained using a variety of morphological, immunological, and genetic criteria, including: nuclear pattern of staining with the fluorochrome Hoechst 33258, appearance of the actin-myosin containing cytoskeleton, presence of fibronectin, and resistance to azaguanine and diphtheria toxin. About 90% of the hybrid cells were viable, that is, capable of division. Changes in the morphology of the hybrid cells, apparently nuclear directed, were observed before cell division occurred. Using the techniques described here, large numbers of interspecies hybrid cells suitable for many types of biochemical analyses can be routinely produced.

Cytoplasmic regulation of two G1-specific temperature-sensitive functions

tsAF8 and ts13 cells are temperature-sensitive (ts) mutants of BHK cells that specifically arrest, at nonpermissive temperature, in the G1 phase of the cell cycle. These two mutants can complement each other. Both cell lines can be made quiescent by serum deprivation (G0). When subsequently stimulated by serum, they can enter S phase at 34 degrees C but not at 39.5 degrees-40.6 degrees C. We have used these mutants to determine whether the nucleus is needed during the G0 leads to S transition for the expression of the G1 ts functions. For this purpose, we fused cytoplasts of G0-tsAF8 with whole ts13 cells in G0, and cytoplasts of G0-ts13 with whole tsAF8 cells in G0. Serum stimulation at the nonpermissive temperature induced DNA synthesis in both types of such fusion products. No DNA synthesis was induced by serum stimulation at the nonpermissive temperature in fusion products constructed between either G0-tsAF8 cytoplasts and whole G0-tsAF8 cells or G0-ts13 cytoplasts and whole G0-ts13 cells. These results demonstrate that the information for these two ts functions, which are required for entry of serum-stimulated cells into the S phase, are already present in the cytoplasm of G0 cells--that is, before serum stimulation commits them to the transition from the nonproliferating to the proliferating state.

Stimulated DNA synthesis in frog nuclei by cytoplasmic extracts of temperature-sensitive mammalian cells

Cytoplasmic extracts of proliferating cells stimulate DNA synthesis in isolated nuclei of Xenopus laevis liver. When tested by the same assay, cytoplasmic extracts of resting cells are completely inactive. When cytoplasmic extracts are prepared from cell cycle-specific temperature-sensitive mutants arrestd in the G1 phase of the cell cycle by the nonpermissive temperature, they also fail to stimulate DNA synthesis in frog nuclei. The results indicate that, to stimulate DNA synthesis in isolated frog nuclei, essentially all information of G1 cells must be present.