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

DNA intercalators differentially affect chromatin structure and DNA replication in Xenopus egg extract

In this paper, we describe a scheme utilizing the Xenopus egg extract system to simultaneously evaluate DNA-interacting drugs as potential anti-cancer agents and gain insights into the mechanisms of drug action. We studied two DNA intercalators, daunomycin (DM), a cancer chemotherapeutic, and ethidium bromide (EtBr), a compound with no reported therapeutic value. Consistent with our earlier report, we find that DM inhibits DNA replication in a concentration-dependent manner. In contrast, EtBr does not inhibit replication over the same concentration range. The environment in which drug-DNA interactions take place is an important determinant of the effect of the drug on DNA replication. While neither intercalator inhibits nuclear membrane assembly nor nuclear protein import, DM does disrupt chromatin structure at very low concentrations, whereas EtBr does not. This system may prove useful for large scale screening of DNA-interacting chemotherapeutic compounds in a cellular milieu.

Nuclear transfer technologies: between successes and doubts

Cloning of mammals by nuclear transfer can lead to the birth of healthy adult animals but more often compromises the development of the reconstructed embryos. A high incidence of fetal and postnatal losses has been observed in several species, revealing the existence of long-lasting effects induced by the nuclear transfer procedures. Remodeling of donor chromatin by the recipient cytoplasm after nuclear transfer is frequently associated with the deregulation of specific genes, and recent observations point to the potential importance of time-dependent DNA methylation events in the occurrence of these alterations. Screening strategies to design nuclear transfer procedures that would mimic the epigenetic remodeling occurring in normal embryos are being designed, and improvement in the efficiency of procedures could imply a pre-conditioning of donor cells. Early mammalian development appears to be rather tolerant to epigenetic abnormalities, raising the possibility that even a fully functional reprogrammed genome may have been subjected to some epigenetic alterations. Bringing nuclear transfer to routine practice requires greater knowledge and understanding of the basic biological processes underlying epigenetic controls of nuclear activities. An important issue at present is to limit the production of those aberrant phenotypes that may result in significant insult to the nature and welfare of animals.

Heterogeneity in nuclear transport does not affect the timing of DNA synthesis in quiescent mammalian nuclei induced to replicate in Xenopus egg extracts

Intact G0 nuclei from quiescent mammalian cells initiate DNA synthesis asynchronously in Xenopus egg extracts, despite exposure to the same concentration of replication factors. This indicates that individual nuclei differ in their ability to respond to the inducers of DNA replication. Since the induction of DNA synthesis requires the accumulation of replication factors by active nuclear transport, any variation in the rate of transport among nuclei could contribute to the variability of DNA replication. Using the naturally fluorescent protein allophycocyanin (APC) coupled with the nuclear localization sequence (NLS) of SV40 T antigen, as a marker of nuclear uptake, we show here that individual G0 nuclei differ in their rate of transport over a range of more than 20-fold. Surprisingly, this variation has no direct influence on the timing or extent of DNA synthesis. Similar results were obtained by monitoring the uptake of nucleoplasmin, a nuclear protein present at high levels in egg extracts. These experiments show that the initiation of DNA synthesis is not driven merely by the accumulation of replication factors to some threshold concentration. Instead, some other explanation is needed to account for the timing of initiation.

Maintenance of nucleolar machineries and pre-rRNAs in remnant nucleolus of erythrocyte nuclei and remodeling in Xenopus egg extracts

The nuclear functions in erythrocytes are almost completely extinct. There is no RNA polymerase I transcription, although a remnant nucleolar structure is still present. The remnant nucleolus of Xenopus laevis erythrocytes maintains a morphologically organized structure, nearly exclusively fibrillar. In this inactive nucleolar remnant, we revealed the presence of a modified form of transcription factor UBF. Several proteins of the processing machinery such as fibrillarin, nucleolin and B23/NO38, snoRNAs U3 and U8, and partially processed preribosomal RNAs colocalized in these remnant structures. Attempts to reprogram these erythrocyte nuclei in Xenopus egg extract showed that import of several nucleolar proteins was induced while the nucleolar remnant was disorganized. UBF became abundant and showed a necklace-like distribution on the decondensed ribosomal genes. Fibrillarin, nucleolin, and snoRNAs U3 and U8, also largely imported from the extract, were associated in large prenuclear bodies scattered in the nucleoplasm. B23/NO38 was present in different small bodies formed only in the most decondensed nuclei. In these remodeled erythrocyte nuclei, there was no imported preribosomal RNA and the initial presence of a residual nucleolar structure containing several partners of ribosome biogenesis was not sufficient to promote reassembly of newly imported nucleolar machineries. These nuclei, which reproduce the early events of nucleogenesis are also transcriptionally silent and thus compare to the early embryonic nuclei of Xenopus laevis.

Lovastatin arrests CHO cells between the origin decision point and the restriction point

Asynchronously growing Chinese hamster ovary (CHO) cells treated with the pro-drug, beta-lactone ring form of lovastatin were arrested in G(1)-phase. Subsequent removal of lovastatin resulted in the synchronous entry of cells into S-phase regardless of the presence of mevalonic acid. Lovastatin-arrested cells contained hypophosphorylated retinoblastoma protein (Rb) and required serum mitogens to enter S-phase after lovastatin removal, indicating that cell-cycle arrest is prior to the restriction point (R-point). However, in contrast to quiescent cells, intact nuclei prepared from lovastatin-arrested cells were competent for DNA replication when introduced into Xenopus egg extracts. Initiation of replication by Xenopus egg cytosol took place specifically within the dihydrofolate reductase (DHFR) origin locus, demonstrating that cells were arrested after the origin decision point (ODP). We conclude that the beta-lactone ring form of lovastatin is an effective reagent with which to synchronize CHO cells between the ODP and R-point, without resulting in the withdrawal of cells from the cell-cycle into a quiescent state.

The replication capacity of intact mammalian nuclei in Xenopus egg extracts declines with quiescence, but the residual DNA synthesis is independent of Xenopus MCM proteins

In eukaryotes, the initiation of DNA synthesis requires the assembly of a pre-replicative complex (pre-RC) at origins of replication. This involves the sequential binding of ORC (origin-recognition-complex), Cdc6 and MCM proteins, a process referred to as licensing. After origin firing, the Cdc6 and MCM proteins dissociate from the chromatin, and do not rebind until after the completion of mitosis, thereby restricting replication to a single round in each cell cycle. Although nuclei normally become licensed for replication as they enter G(1), the extent to which the license is retained when cells enter the quiescent state (G(0)) is controversial. Here we show that the replication capacity of nuclei from Swiss 3T3 cells, in Xenopus egg extracts, is not lost abruptly with the onset of quiescence, but instead declines gradually. The decline in replication capacity, which affects both the number of nuclei induced to replicate and their subsequent rate of DNA synthesis, is accompanied by a fall in the level of chromatin-bound MCM2. When quiescent nuclei are incubated in egg extracts, they do not bind further MCMs unless the nuclei are first permeabilized. The residual replication capacity of intact nuclei must therefore be dependent on the remaining endogenous MCMs. Although high levels of Cdk activity are known to block MCM binding, we show that the failure of intact nuclei in egg extracts to increase their bound MCMs is not due to their uptake and accumulation of Cdk complexes. Instead, the failure of binding must be due to exclusion of some other binding factor from the nucleus, or to the presence within nuclei of an inhibitor of binding other than Cdk activity. In contrast to the situation in Xenopus egg extracts, following serum stimulation of intact quiescent cells, the level of bound MCMs does increase before the cells reach S phase, without any disruption of the nuclear envelope.

DNA replication in quiescent cell nuclei: regulation by the nuclear envelope and chromatin structure

Quiescent nuclei from differentiated somatic cells can reacquire pluripotence, the capacity to replicate, and reinitiate a program of differentiation after transplantation into amphibian eggs. The replication of quiescent nuclei is recapitulated in extracts derived from activated Xenopus eggs; therefore, we have exploited this cell-free system to explore the mechanisms that regulate initiation of replication in nuclei from terminally differentiated Xenopus erythrocytes. We find that these nuclei lack many, if not all, pre-replication complex (pre-RC) proteins. Pre-RC proteins from the extract form a stable association with the chromatin of permeable nuclei, which replicate in this system, but not with the chromatin of intact nuclei, which do not replicate, even though these proteins cross an intact nuclear envelope. During extract incubation, the linker histones H1 and H1(0) are removed from erythrocyte chromatin by nucleoplasmin. We show that H1 removal facilitates the replication of permeable nuclei by increasing the frequency of initiation most likely by promoting the assembly of pre-RCs on chromatin. These data indicate that initiation in erythrocyte nuclei requires the acquisition of pre-RC proteins from egg extract and that pre-RC assembly requires the loss of nuclear envelope integrity and is facilitated by the removal of linker histone H1 from chromatin.

Mechanisms and control of embryonic genome activation in mammalian embryos

Activation of transcription within the embryonic genome (EGA) after fertilization is a complex process requiring a carefully coordinated series of nuclear and cytoplasmic events, which collectively ensure that the two parental genomes can be faithfully reprogrammed and restructured before transcription occurs. Available data indicate that inappropriate transcription of some genes during the period of nuclear reprogramming can have long-term detrimental effects on the embryo. Therefore, precise control over the time of EGA is essential for normal embryogenesis. In most mammals, genome activation occurs in a stepwise manner. In the mouse, for example, some transcription occurs during the second half of the one-cell stage, and then a much greater phase of genome activation occurs in two waves during the two-cell stage, with the second wave producing the largest onset of de novo gene expression. Changes in nuclear structure, chromatin structure, and cytoplasmic macromolecular content appear to regulate these periods of transcriptional activation. A model is presented in which a combination of cell cycle-dependent events and both translational and posttranslational regulatory mechanisms within the cytoplasm play key roles in mediating and regulating EGA.

Replication of nuclei from cycling and quiescent mammalian cells in 6-DMAP-treated Xenopus egg extract

Nuclear membrane permeabilization is required for replication of quiescent (G0) cell nuclei in Xenopus egg extract. We now demonstrate that establishment of replication competence in G0 nuclei is dependent upon a positive activity present in the soluble egg extract. Our hypothesis is that G0 nuclei lose the license to replicate following growth arrest and that this positive activity is required for relicensing DNA for replication. To determine if G0 nuclei contain licensed DNA, we used the protein kinase inhibitor, 6-dimethylaminopurine (6-DMAP), to prepare egg extracts that are devoid of licensing activity. Intact nuclei, isolated from mammalian cells synchronized in G1-phase (licensed), G2-phase (unlicensed), and G0 were permeabilized and assayed for replication in 6-DMAP-treated and untreated extracts supplemented with [alpha-32P]dATP or biotinylated-dUTP. Very little radioactivity was incorporated into nascent DNA in each nuclear population; however, nearly all nuclei in each population incorporated biotin in 6-DMAP extract. The pattern of biotin incorporation within these nuclei was strikingly similar to the punctate pattern observed within nuclei incubated in aphidicolin-treated extract, suggesting that initiation events occur within most replication factories in 6-DMAP extract. However, density substitution and alkaline gel analyses indicate that the incorporated biotin within these nuclei arises from a small number of active origins which escape 6-DMAP inhibition. We conclude that 6-DMAP-treated egg extract cannot differentiate licensed from unlicensed mammalian somatic cell nuclei and, therefore, cannot be used to determine the "licensed state" of G0 nuclei using the assays described here.