In vitro translation in reovirus- and poliovirus-infected cell extracts. Effects of anti-cap binding protein monoclonal antibody

Cell Entry-Independent Role for the Reovirus μ1 Protein in Regulating Necroptosis and the Accumulation of Viral Gene Products

The reovirus outer capsid protein μ1 regulates cell death in infected cells. To distinguish between the roles of incoming, capsid-associated, and newly synthesized μ1, we used small interfering RNA (siRNA)-mediated knockdown. Loss of newly synthesized μ1 protein does not affect apoptotic cell death in HeLa cells but enhances necroptosis in L929 cells. Knockdown of μ1 also affects aspects of viral replication. We found that, while μ1 knockdown results in diminished release of infectious viral progeny from infected cells, viral minus-strand RNA, plus-strand RNA, and proteins that are not targeted by the μ1 siRNA accumulate to a greater extent than in control siRNA-treated cells. Furthermore, we observed a decrease in sensitivity of these viral products to inhibition by guanidine hydrochloride (GuHCl) (which targets minus-strand synthesis to produce double-stranded RNA) when μ1 is knocked down. Following μ1 knockdown, cell death is also less sensitive to treatment with GuHCl. Our studies suggest that the absence of μ1 allows enhanced transcriptional activity of newly synthesized cores and the consequent accumulation of viral gene products. We speculate that enhanced accumulation and detection of these gene products due to μ1 knockdown potentiates receptor-interacting protein 3 (RIP3)-dependent cell death.IMPORTANCE We used mammalian reovirus as a model to study how virus infections result in cell death. Here, we sought to determine how viral factors regulate cell death. Our work highlights a previously unknown role for the reovirus outer capsid protein μ1 in limiting the induction of a necrotic form of cell death called necroptosis. Induction of cell death by necroptosis requires the detection of viral gene products late in infection; μ1 limits cell death by this mechanism because it prevents excessive accumulation of viral gene products that trigger cell death.

Synthesis and Translation of Viral mRNA in Reovirus-Infected Cells: Progress and Remaining Questions

At the end of my doctoral studies, in 1988, I published a review article on the major steps of transcription and translation during the mammalian reovirus multiplication cycle, a topic that still fascinates me 30 years later. It is in the nature of scientific research to generate further questioning as new knowledge emerges. Our understanding of these fascinating viruses thus remains incomplete but it seemed appropriate at this moment to look back and reflect on our progress and most important questions that still puzzle us. It is also essential of being careful about concepts that seem so well established, but could still be better validated using new approaches. I hope that the few reflections presented here will stimulate discussions and maybe attract new investigators into the field of reovirus research. Many other aspects of the viral multiplication cycle would merit our attention. However, I will essentially limit my discussion to these central aspects of the viral cycle that are transcription of viral genes and their phenotypic expression through the host cell translational machinery. The objective here is not to review every aspect but to put more emphasis on important progress and challenges in the field.

Chapter 7 Orthomyxovirus infections

The earth is a unity for influenza A virus in a manner not yet found for probably any other parasite and epidemics occur in all inhabited parts of the globe regardless of latitude, longitude, altitude, climate, rainfall, temperature, humidity, race and sex. Influenza A is the classic pandemic virus infection of man and influenza B virus also can cause sharp outbreaks, resulting in significant mortality. An overwhelming amount of data has accumulated on the biochemistry, cell biology, and epidemiology of influenza, but prospects of control of epidemics in the near future are dim. Meanwhile, a holding operation can be achieved using inactivated vaccine and rimantadine (100 mg/daily) in special risk groups in the population until new more effective vaccines and broad spectrum antivirals (active against influenza A and B virus) are developed. Research work is centered on biotechnology to produce immunogenic peptides and proteins and more logical searches for antivirals using amino acid sequence data and also virus specific enzymes such as the virion transcriptase as targets.

The 5' leader of the mRNA encoding the mouse neurotrophin receptor TrkB contains two internal ribosomal entry sites that are differentially regulated

A single internal ribosomal entry site (IRES) in conjunction with IRES transactivating factors (ITAFs) is sufficient to recruit the translational machinery to a eukaryotic mRNA independent of the cap structure. However, we demonstrate that the mouse TrkB mRNA contains two independent IRESes. The mouse TrkB mRNA consists of one of two 5′ leaders (1428 nt and 448 nt), both of which include the common 3′ exon (Ex2, 344 nt). Dicistronic RNA transfections and in vitro translation of monocistronic RNA demonstrated that both full-length 5′ leaders, as well as Ex2, exhibit IRES activity indicating the IRES is located within Ex2. Additional analysis of the upstream sequences demonstrated that the first 260 nt of exon 1 (Ex1a) also contains an IRES. Dicistronic RNA transfections into SH-SY5Y cells showed the Ex1a IRES is constitutively active. However, the Ex2 IRES is only active in response to retinoic acid induced neural differentiation, a state which correlates with the synthesis of the ITAF polypyrimidine tract binding protein (PTB1). Correspondingly, addition or knock-down of PTB1 altered Ex2, but not Ex1a IRES activity in vitro and ex vivo, respectively. These results demonstrate that the two functionally independent IRESes within the mouse TrkB 5′ leader are differentially regulated, in part by PTB1.

Ribosome topography in baby hamster kidney cells infected with Sindbis and vesicular stomatitis viruses

The topography of polysomal ribosomes in mock-infected and in Sindbis virus- and vesicular stomatitis virus-infected BHK cells was investigated using a double, radioactive labelling technique. Ribosomal proteins in intact polysomes were surface labelled by reductive methylation using [14C]formaldehyde. Following removal of ribosomal RNA, proteins were denatured in 6 M guanidine and labelled with [3H]borohydride. Labelled ribosomal proteins were separated by electrophoresis in two-dimensional gels and the 3H/14C ratio for each ribosomal protein was taken as an index of its relative surface exposure in intact ribosomes. Comparison of the ratios for individual ribosomal proteins in Sindbis virus-infected vs. control polysomes indicated that proteins L7, L8, L17, L26 and S19 became more 'buried' and others such as L4, L29, L36, S2 and S26 became more 'exposed' in infected cells. Most of the topographical alterations occurred in the large ribosomal subunit. In contrast, infection of BHK cells with vesicular stomatitis virus induced little or no topographical alteration.

Messenger ribonucleoproteins of cells infected by simian virus 40 contain large T-antigen

In monkey or mouse cells undergoing lytic or transforming infection with simian virus 40, about 10% of large tumor antigen (T-antigen) molecules were consistently present in the cytoplasm and 90% in the nucleus. The bulk of cytoplasmic large T-antigen cosedimented in linear sucrose gradients with polyribosomes (150-500 S) and with messenger ribonucleoproteins sedimenting within 20-80 S. As determined by centrifugation in discontinuous sucrose gradients, its apparent density (1.2-1.3 g/ml) corresponded to that of ribonucleoproteins. In contrast, the bulk of nuclear T-antigen sedimented between 5-20 S and its apparent density (1.1 g/ml) corresponded to that of free protein. Nuclear T-antigen added before cell fractionation did not bind to cytoplasmic constituents. After dissociation of purified polyribosomes with puromycin/KCl or EDTA, cytoplasmic large T-antigen cosedimented with the released messenger ribonucleoproteins containing poly(A)-rich messenger RNA. Upon hydrolysis of the RNA with RNase A, large T-antigen exhibited the sedimentation properties and density of free protein. The results suggest that cytoplasmic large T-antigen is associated with messenger ribonucleoproteins.

Association of cap-binding protein with eucaryotic initiation factor 3 in initiation factor preparations from uninfected and poliovirus-infected HeLa cells

Extracts from poliovirus-infected HeLa cells are unable to translate vesicular stomatitis virus or cellular mRNAs in vitro, probably reflecting the poliovirus-induced inhibition of host cell protein synthesis which occurs in vivo. Crude initiation factors from uninfected HeLa cells are able to restore translation of vesicular stomatitis virus mRNA in infected cell lysates. This restoring activity separates into the 0 to 40% ammonium sulfate fractional precipitate of ribosomal salt wash. Restoring activity is completely lacking in the analogous fractions prepared from poliovirus-infected cells. The 0 to 40% ammonium sulfate precipitates from both uninfected and infected cells contain eucaryotic initiation factor 3 (eIF-3), eIf-4B, and the cap-binding protein (CBP), which is detected by means of a cross-linking assay, as well as other proteins. The association of eIF-3 and cap binding protein was examined. The 0 to 40% ammonium sulfate precipitate of ribosomal salt wash from uninfected and infected cells was sedimented in sucrose gradients. Each fraction was examined for the presence of eIF-3 antigens by an antibody blot technique and for the presence of the CBP by cross-linking to cap-labeled mRNAs. From uninfected cells, a major proportion of the CBP cosedimented with eIF-3; however, none of the CBP from infected cells sedimented with eIF-3. The results suggest that the association of the CBP with eIF-3 into a functional complex may have been disrupted during the course of poliovirus infection.