VIRAL INHIBITION IN THE METAPHASE-ARREST CELL

Vaccinia virus RNA polymerase associated with nuclei of infected HeLa cells

Though vaccinia virus DNA and RNA replication take place predominantly in the cytoplasm of an infected cell, virus formation requires the presence of a functional nucleus in a yet undefined manner. When the nuclei from cells infected for 3 h are isolated and purified, they are found to synthesize five times more RNA in vitro than do corresponding nuclei from noninfected cells. Fifty percent of the RNA synthesized in vitro by nuclei from infected cells is vaccinia specific, and this vaccinia RNA synthesis is resistant to alpha-amanitin concentrations up to 100 micrograms/ml. Furthermore, when the RNA polymerase activities of these nuclei are separated on DEAE-Sephadex columns, 56% of the total nuclear enzyme activity is found to be the vaccinia-specific RNA polymerase known to be alpha-amanitin resistant. The nucleus associated vaccinia RNA polymerase represents 18% of the total cellular vaccinia RNA polymerase. This synthesis of vaccinia RNA in the nucleus may explain the nuclear requirement for vaccinia virus maturation.

Evidence of heterogeneity of protein-turnover states in cultured cells

Previous studies on L-cell cultures [Amenta & Sargus (1979) Biochem. J. 182, 847--859] have suggested: (a) that degradation of slow-turnover proteins occurs in a distinct cell state (D-state); (b) that cells randomly enter the D-state with a first-order transition constant, rapidly degrade cell protein, and return to a quiescent G0-state. In the present study we have tested the hypothesis that the putative D-state exists as a substate within A-state (non-replicating) fibroblasts. Rat-embryo fibroblasts were prelabelled with [14C]leucine and [3H]thymidine, 'chased' for 24 h, and then placed in fresh growth medium containing either vinblastine (10 microM) or colchicine (25 microM) for three successive 24 h periods. Cells trapped in mitosis were separated from the residual non-replicating cells and rates of protein synthesis, degradation and net accumulation were measured in both populations. We observed that significant protein degradation occurred only in the non-replicating population, although both populations showed equally high rates of protein synthesis induced by fresh growth medium. These data support the hypothesis that degradation of slow-turnover protein is heterogeneous, occurring only in A-state cells. A model that proposes a separate D-state within G0-phase successfully accounts for these observations and previous reports on this cell line [Amenta, Sargus & Baccino (1978) J. Cell. Physiol. 97, 267--283] showing no differences in degradation of the slow-turnover protein pool in growth-stimulated and stationary-phase fibroblast cultures.

Influence of poliovirus infection on S-phase and mitosis of the host cell

Hep-2 cells were synchronized by a double thymidine block and infected with poliovirus type I (Mahoney) in hourly intervals after release from the second thymidine block. The S-phase is not prevented by a poliovirus infection but, with cells infected 0-4 hours after release, an increase of its duration is found. With an infection 5 hours and later after release, the duration of the S-phase is not different from that of an uninfected, synchronized control culture. DNA synthesis itself is slower early in S-phase and gets inhibited up to 75 per cent late in S-phase. All cultures show the first signs of a CPE 3.5 hours p.i. and, in spite of CPE, the cells continue to synthesize DNA. In infected cells a slightly higher peak of mitotic cells compared to control cultures is found. The time point of the mitotic peak is dependant of the time of infection and seems no longer controlled by the cell cycle. The mitotic indices are similar for all cultures infected at different times after release. When the cells are infected early after release CPE appears before mitosis and prevents the cells from entering mitosis. Cells which are infected towards the end of the S-phase finish mitosis normally before they exhibit characteristics of CPE. Extent and kinetics of poliovirus RNA synthesis and yield of virus progeny are not altered by the cell cycle.

Vaccinia virus infection of HeLa cells. I. Synthesis of vaccinia DNA in host cell nuclei

The replication of vaccinia virus is thought to take place exclusively in the cytoplasm of host cells. However, using DNA-DNA hybridization techniques, it can be shown that a significant fraction of the synthesis of vaccinia DNA takes place in the nucleus as well as the cytoplasm. The (3H) thymiding pulse-labeled vaccinia DNA synthesized in the nucleus reaches a maximum at about 3 h after infection, corresponding to the time of maximal DNA synthesis in infected cells. At this time host DNA synthesis drops to about 25% of the rate of the uninfected cells. Even with short labeling times (2 min) the nucleus is found to contain 60% of the incorporated (3H)thymidine, much of which is in vaccinia DNA. Prior inhibition of host nuclear DNA synthesis with mitomycin C, followed by removal of the antibiotic causes a subsequent inhibition of vaccinia DNA synthesis and complete suppression of mature virus. Purified nuclei, isolated from vaccinia-infected cells, also synthesize vaccinia DNA in vitro. Over 90% of the DNA synthesized in vitro by isolated nuclei contain vaccinia-specific sequences.

Sedimentation changes of L cells in a density gradient early after infection with vaccinia virus

By 2 h postinfection, LM cells infected with vaccinia virus show a shift in their distribution when separated on a Ficoll density gradient. This shift is dependent on both time and the multiplicity of infection and is due, at least in part, to an increase in cell size. Those cells which do shift in position in the gradient represent infected members of the population. Physical changes induced in virus-infected cells can be utilized for studying early events in virus replication.

Virus replication, cytopathology, and lysosomal enzyme response of mitotic and interphase Hep-2 cells infected with poliovirus

Mitotic Hep-2 cells, selected by the PEL (colloidal silica) density gradient method and held in mitosis with Colcemid, are readily infected by poliovirus type I (Mahoney). They produce and release the same amount of virus as interphase, random-growing cells. In contrast to interphase cells, mitotic cells show no detectable virus-induced cytopathic effect at the light microscopy level and only slight alterations, consisting of small clusters of vacuoles, at the electron microscopy level. Mitotic cells contain the same total amount of lysosomal enzymes per cell as interphase cells, but they display no redistribution of lysosomal enzymes during the virus infection as interphase cells do. This supports the view that lysosomal enzyme redistribution is associated with the cytopathic effect in poliovirus infection but shows that virus synthesis and release is not dependent on either the cytopathic effect or lysosomal enzyme release. The possible reasons for the lack of cytopathic effect in mitotic cells are discussed.

Delay of mengovirus-induced cytopathology in mitotic L-cells

The time course and extent of mengovirus production were the same in metaphase-arrested and interphase L-cells, yet the expression of cytopathology was delayed several hours in metaphase cells.

Vaccinia virus infection of synchronized pig kidney cells

Vaccinia virus replication was studied in pig kidney (PK-15) cells synchronized by excess thymidine treatment. It was found that virus replication with concomitant inhibition of mitosis can occur at any period in the life cycle of the cell except for the narrow span of time from late prophase through telophase. Cells infected at this time continue to divide, and vaccinia does not replicate until cell division is complete.

Intrinsic interference: a new type of viral interference

The hemadsorption-negative plaque test has revealed a new type of viral interference, termed intrinsic interference. Several unrelated types of noncytopathic viruses were shown to induce in infected host cells a state of interference unique in being directed solely against superinfection by Newcastle disease virus (NDV). The NDV-refractory state arises only in those individual cells of a population actually infected by the inducing virus, and presumably results from the action of a protein(s) coded for by the viral genome. Thus, intrinsic interference differs fundamentally from that mediated by an extrinsic protein detectable under conditions favoring resistance to a broad spectrum of viruses and characteristic of interference induced by interferon, the latter being coded for by the cell genome. Intrinsic interference is defined as a viral genome-induced cellular state of resistance to challenge by high multiplicities of NDV, coexistent with a state of susceptibility to a broad spectrum of other viruses, similarly tested at high multiplicities. The capacity to induce intrinsic interference was demonstrated with rubella virus, Sindbis virus (arbovirus, group A), West Nile virus (arbovirus, group B), poliovirus (MEF, type 2), the lactic dehydrogenase virus (Riley's agent), and an unidentified nonhemadsorbing, noncytopathic adventitious virus. A state of intrinsic interference was also observed in the V5 line of mouse cells carrying a murine leukemia virus, probably resulting from some heretofore unsuspected contaminating virus. The molecular basis for intrinsic interference is not known, but it appears to involve a step in the NDV growth cycle beyond that of viral attachment, entry, and eclipse.

Ribonucleic acid and protein synthesis in mitotic HeLa cells

HeLa cells arrested in mitosis were obtained in large numbers, with only very slight interphase cell contamination, by employing the agitation method of Terasima and Tolmach, and Robbins and Marcus. Protein synthesis and RNA synthesis were almost completely suppressed in mitotic cells. Active polyribosomes were nearly absent in mitotic cells as compared with interphase cells treated in the same way. Cell-free protein synthesis and RNA polymerase activity were also greatly depressed in extracts of metaphase cells. The deoxyribonucleoprotein (DNP) of condensed chromosomes from mitotic cells was less efficient as a template for Escherichia coli RNA polymerase than was DNP from interphase cells, although isolated DNA from both sources was equally active as a primer. Despite very poor endogenous amino acid incorporation by extracts of metaphase cells, polyuridylate stimulated phenylalanine incorporation by a larger factor in mitotic cell extracts than it did in interphase cell extracts. These results suggest that RNA synthesis is suppressed in mitotic cells because the condensed chromosomes cannot act as a template, and that protein synthesis is depressed at least in part because messenger RNA becomes unavailable to ribosomes. This conclusion was supported by the demonstration that cells arrested in metaphase supported multiplication of normal yields of poliovirus, thereby showing that the mitotic cell is capable of considerable synthesis of RNA and protein.

MITOTICALLY SYNCHRONIZED MAMMALIAN CELLS: A SIMPLE METHOD FOR OBTAINING LARGE POPULATIONS

Eliminating most of the ionic calcium from growth medium does not affect cell growth, but it allows simple and preferential detachment of HeLa cells in mitosis from nearly confluent monolayers.