Reovirus infected

Dryden, K. A., Wang, G., Yeager, M., Nibert, M. L., Coombs, K. M., Furlong, D. B., Fields, B. N., and Baker, T. S. (1993). Early steps in reovirus infection are associated with dramatic changes in supramolecular structure and protein conformation Analysis of virions and subviral particles by cryoelectron microscopy and image reconstruction./. Cell Biol. 122, 1023-1041. 

One question that deserves attention is the fact that, at least in the case of some virus-cell systems, viral RNA translation is inhibited preferentially over host mRNA translation, for example, in reovirus-infected L cells (Gupta et al., 1974). An interesting possibility was raised by Nilsen and Baglioni (1979). They showed that, in extracts of interferon-treated cells, VSV mRNA hybridized with poly (U) at its poly (A) tail or EMC RNA hybridized with poly (I) at its poly (C) tract are more rapidly degraded than the corresponding control mRNAs. They proposed that, in infected, interferon-treated cells, activation of the endoribonuclease takes place near the replicative intermediate of RNA viruses, because the dsRNA moiety therein promotes the formation of (2 -5 )oligo (A) in its vicinity. As a result, the viral mRNA portion in the replicative intermediate may be more sensitive to degradation than host mRNA. 

From these data, it is tempting to conclude that the inactivation of cap-dependent translation which seems to occur in poliovirus-infected HeLa cells, and perhaps in reovirus-infected L cells, also occurs in neuroblastoma cells infected with SFV. Although the relevance of a redirection of the protein-synthesizing system towards noncapped mRNAs is obvious for uncapped poliovirus or for uncapped reovirus mRNAs, the rationale for SFV-infected cells is obscured because SFV mRNAs, when isolated from infected cells, appear to be capped (Pettersson et al., 1980). In addition, further analyses of initiation factors from SFV-infected cells suggest that the mechanisms operative in the shift in specificity might be different from the biochemical lesion(s) induced by poliovirus. 

Detjen, B. M., Walden, W. E., and Thach, R. E., 1982, Translational specificity in reovirus-infected mouse fibroblasts, J. Biol. Chem. 257 9855. 

Pursuing similar types of experiments with reovirus-infected L cells, Skup et al. studied the ability of CBP to restore translation of capped reovirus mRNAs. They found that CBP could partially restore activity to lysates from infected cells (Zarbl and Millward, 1983). Zarbl and Millward, however, point out that inactivation of CBP does not explain why uncapped reovirus mRNAs can be translated in infected cells because, unlike other uncapped mRNAs which can be translated in lysates from uninfected cells, uncapped reovirus mRNAs cannot be translated under these conditions (Skup and Millward, 1980a, >). Therefore, they propose that an additional factor, presumably viral, is necessary for the translation of reovirus uncapped mRNAs. In order to determine whether this is the case, Skup and Millward have examined the effect of antireovirus antibodies on the translation of capped and uncapped reovirus mRNAs in vitro. The antiserum specifically inhibits the translation of uncapped viral... 

In contrast to Millward and his colleagues, Detjen et al. (1982) find that protein synthesis in SC-1 cells remains cap dependent throughout infection and do not find a transition from cap-dependent to cap-independent translation in reovirus-infected L cells. These findings are consistent with their proposed mechanism of translation control by mRNA competition for a discriminatory factor. They suggest that a partial explanation for the difference of their results with those of Skup and Millward (1980u,Z>) may be the difference in experimental technique. They used m GTP inhibition to assess the extent of capping of mRNA species, which measures only mRNAs actively translated. Thus, their method would not detect the presence of uncapped mRNAs unless they were capable of translation. Thus, they cannot exclude the possibility that uncapped mRNAs are present but not translatable for some unclear reason. Millward and his coworkers also find that the translation of late reovirus mRNA is sensitive to m GTP but do not have an explanation for this observation (Zarbl and Millward, 1983). Clearly, further studies are necessary to explain the conflicts in the data reported by the laboratories of Mill-ward and Thach. 

With respect to cellular RNA synthesis, virtually no inhibition of host transcription has been observed following the infection of L cells with reovirus type 3 (Gomatos and Tamm, 1963 Kudo and Graham, 1965 Sharpe and Fields, 1982). Host mRNAs are present in type 3 reovirus-infected L cells late in the infectious cycle, although they are not translated, indicating host mRNA stability in the infected cell (Skup et ai, 1981). If the mechanism of reovirus inhibition of cellular protein synthesis does indeed involve a shift of the host translational machinery from cap dependence to cap independence, then the inhibition of protein synthesis does not require that reovirus induce an inhibition of host mRNA synthesis since all host mRNAs are capped. 

Fig. 4. Reovirus-infected monkey kidney CV-1 cells at 48 hr postinfection with reovirus type 3. Cells were stained with rabbit antireovirus serum and fluorescein-conjugated goat and rabbit serum according to the method of Sharpe et al. (1982). Cytoplasmic inclusions are represented by the numerous globular white areas within the cytoplasm. Note the gradation of size of inclusions, from small to large, as the inclusions approach the nucleus (bar = 20 fxm). From Sharpe et al. (1982), by permission of Virology.

In contrast to the marked disruption of intermediate filaments, reovirus infection did not disrupt microtubule organization. Antibodies to tubulin visualized microtubules coursing through regions of the cytoplasm containing viral factories, without interruption or distortion. These findings are consistent with the electron-microscopic studies that indicate that reovirions are aligned on parallel arrays of microtubules within viral factories (Dales et al., 1965) and can bind to microtubules in vitro (Babiss et al., 1979). 

Since colchicine treatment of reovirus-infected cells does not reduce viral yield, whether microtubules play a role in viral growth is uncertain (Spendlove et al., 1964). Colchicine treatment of virally infected cells, however, does alter the morphology of viral inclusions. Only small inclusions located at the cell periphery are seen in colchicine-treated infected cells. The large perinuclear inclusions usually observed in reovirus-infected cells are not seen. Thus, microtubules may be involved in the coalescence of viral inclusions and in inclusion movement toward the cell nucleus. 

Reovirus infection not only produces a disruption of intermediate filaments, but also leads to a disorganization of mitochondrial distribution (Sharpe et al., 1982). Vizualized with the fluorescent probe Rhodamine 123 (Johnson et al., 1980 Walsh et al., 1979), mitochondria have a characteristic discontinuous distribution in the CV-1 cell cytoplasm. Reovirus infections result in the aggregation of mitochondria around the nucleus with only occasional mitochondria present at the cell periphery. Mitochondria are not present within viral inclusions. Although reovirus infection affects mitrochondrial distribution, whether reovirus infection alters mitochondrial function is uncertain. Johnson et al. (1981) showed that the accumulation of Rhodamine 123 by mitochondria reflects the transmembrane potential. The accumulation of Rhodamine 123 is similar in infected and unin-... 

Ahmed, R., and Fields, B. N., 1982, Role of the S4 gene in establishment of persistent reovirus infection in L cells, Cell 28 605. 

Chaly, N., Johnstone, M., and Hand, R., 1980, Alterations in nuclear structure and function in reovirus-infected cells, Clin. Invest. Med. 2 141. 

Ensminger, W. D., and Tamm, I., 1969a, Cellular DNA and protein synthesis in reovirus-infected cells. Virology 39 357. 

Finberg, R., Weiner, H. L., Fields, B. N., Benacerraf, B., and Burakoff, S. J., 1979, Generation of cytolytic T lymphocytes after reovirus infection Role of SI gene, Proc. Natl. Acad. Sci. USA 76 442. 

Gomatos, P. J., and Tamm, L, 1963, Macromolecular synthesis in reovirus-infected cells, Biochim. Biophys. Acta 72 651. 

Hand, R., and Tamm, I., 1974, Initiation of DNA synthesis in mammalian cells and its inhibition by reovirus infection, J. Mol. Biol. 82 175. 

SiLVERSTEiN, S. C., ScHUR, P. H. Immunofluorescent localization of double-stranded RNA in reovirus-infected cells. Virology 41, 564-566 (1970). 

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