Reovirus capsid proteins sigma 3 and mu 1: interactions that influence viral entry, assembly, and translational control

Proteases facilitate the endosomal escape of porcine epidemic diarrhea virus during entry into host cells

Exogenous and endogenous proteases play important roles in porcine epidemic diarrhea virus (PEDV) entry and replication. The roles of proteases in the viral endosomal escape and replication using trypsin (KD) or elastase (AA)-adapted US PEDV strains were studied. While PEDV KD and AA require different exogenous protease for efficient replication in cells, PEDV KD was more dependent on the protease than PEDV AA. There was no marked difference in viral trafficking between them during the entry events. Both PEDV were observed in the endosomes with or without protease at 1 h after virus inoculation. With protease, viral signals in the endosomes disappeared after 4 h, and newly synthesized viral proteins were detected in the ER after 6 h. However, without protease, viruses remained in the endosomes up to 24 h, which correlated with limited virus replication. Inhibitors of cathepsins, endogenous proteases, significantly reduced the replication of both PEDV by interfering with the viral endosomal escape.

How Many Mammalian Reovirus Proteins are involved in the Control of the Interferon Response?

As with most viruses, mammalian reovirus can be recognized and attacked by the host-cell interferon response network. Similarly, many viruses have developed resistance mechanisms to counteract the host-cell response at different points of this response. Reflecting the complexity of the interferon signaling pathways as well as the resulting antiviral response, viruses can-and often have-evolved many determinants to interfere with this innate immune response and allow viral replication. In the last few years, it has been evidenced that mammalian reovirus encodes many different determinants that are involved in regulating the induction of the interferon response or in interfering with the action of interferon-stimulated gene products. In this brief review, we present our current understanding of the different reovirus proteins known to be involved, introduce their postulated modes of action, and raise current questions that may lead to further investigations.

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.

Potential for Improving Potency and Specificity of Reovirus Oncolysis with Next-Generation Reovirus Variants

Viruses that specifically replicate in tumor over normal cells offer promising cancer therapies. Oncolytic viruses (OV) not only kill the tumor cells directly; they also promote anti-tumor immunotherapeutic responses. Other major advantages of OVs are that they dose-escalate in tumors and can be genetically engineered to enhance potency and specificity. Unmodified wild type reovirus is a propitious OV currently in phase I–III clinical trials. This review summarizes modifications to reovirus that may improve potency and/or specificity during oncolysis. Classical genetics approaches have revealed reovirus variants with improved adaptation towards tumors or with enhanced ability to establish specific steps of virus replication and cell killing among transformed cells. The recent emergence of a reverse genetics system for reovirus has provided novel strategies to fine-tune reovirus proteins or introduce exogenous genes that could promote oncolytic activity. Over the next decade, these findings are likely to generate better-optimized second-generation reovirus vectors and improve the efficacy of oncolytic reotherapy.

Sequences of avian reovirus M1, M2 and M3 genes and predicted structure/function of the encoded mu proteins

We report the first sequence analysis of the entire complement of M-class genome segments of an avian reovirus (ARV). We analyzed the M1, M2 and M3 genome segment sequences, and sequences of the corresponding muA, muB and muNS proteins, of two virus strains, ARV138 and ARV176. The ARV M1 genes were 2,283 nucleotides in length and predicted to encode muA proteins of 732 residues. Alignment of the homologous mammalian reovirus (MRV) mu2 and ARV muA proteins revealed a relatively low overall amino acid identity ( approximately 30%), although several highly conserved regions were identified that may contribute to conserved structural and/or functional properties of this minor core protein (i.e. the MRV mu2 protein is an NTPase and a putative RNA-dependent RNA polymerase cofactor). The ARV M2 genes were 2158 nucleotides in length, encoding predicted muB major outer capsid proteins of 676 amino acids, more than 30 amino acids shorter than the homologous MRV mu1 proteins. In spite of the difference in size, the ARV/MRV muB/mu1 proteins were more conserved than any of the homologous proteins encoded by other M- or S-class genome segments, exhibiting percent amino acid identities of approximately 45%. The conserved regions included the residues involved in the maturation- and entry- specific proteolytic cleavages that occur in the MRV mu1 protein. Notably missing was a region recently implicated in MRV mu1 stabilization and in forming "hub and spokes" complexes in the MRV outer capsid. The ARV M3 genes were 1996 nucleotides in length and predicted to encode a muNS non-structural protein of 635 amino acids, significantly shorter than the homologous MRV muNS protein, which is attributed to several substantial deletions in the aligned ARV muNS proteins. Alignments of the ARV and MRV muNS proteins revealed a low overall amino acid identity ( approximately 25%), although several regions were relatively conserved.

Translational control of viral gene expression in eukaryotes

As obligate intracellular parasites, viruses rely exclusively on the translational machinery of the host cell for the synthesis of viral proteins. This relationship has imposed numerous challenges on both the infecting virus and the host cell. Importantly, viruses must compete with the endogenous transcripts of the host cell for the translation of viral mRNA. Eukaryotic viruses have thus evolved diverse mechanisms to ensure translational efficiency of viral mRNA above and beyond that of cellular mRNA. Mechanisms that facilitate the efficient and selective translation of viral mRNA may be inherent in the structure of the viral nucleic acid itself and can involve the recruitment and/or modification of specific host factors. These processes serve to redirect the translation apparatus to favor viral transcripts, and they often come at the expense of the host cell. Accordingly, eukaryotic cells have developed antiviral countermeasures to target the translational machinery and disrupt protein synthesis during the course of virus infection. Not to be outdone, many viruses have answered these countermeasures with their own mechanisms to disrupt cellular antiviral pathways, thereby ensuring the uncompromised translation of virion proteins. Here we review the varied and complex translational programs employed by eukaryotic viruses. We discuss how these translational strategies have been incorporated into the virus life cycle and examine how such programming contributes to the pathogenesis of the host cell.