Mammalian Reovirus M3 Gene Sequences and Conservation of Coiled-Coil Motifs near the Carboxyl Terminus of the μNS Protein

AbstractNucleotide sequences of the mammalian orthoreovirus (reovirus) type 1 Lang and type 2 Jones M3 gene segments were newly determined. The nucleotide sequence of the reovirus type 3 Dearing M3 segment also was determined to compare with a previously reported M3 sequence for that isolate. Comparisons showed Lang and Dearing M3 to be more closely related than either was to Jones M3, consistent with previous findings for other reovirus gene segments. The μNS protein sequences deduced from each M3 segment were shown to be related in a similar pattern as the respective nucleotide sequences and to contain several regions of greater or less than average variability among the three isolates. Identification of conserved methionine codons near the 5′ ends of the Lang, Jones, and Dearing M3 plus strands lent support to the hypothesis that μNSC, a smaller protein also encoded by M3, arises by translation initiation from a downstream methionine codon within the same open reading frame as μNS. Other analyses of the deduced protein sequences indicated that regions within the carboxyl-terminal third of μNS and μNSC from each isolate have a propensity to form α-helical coiled coils, most likely coiled-coil dimers. The new sequences will augment further studies on μNS and μNSC structure and function

Comparisons of the M1 genome segments and encoded μ2 proteins of different reovirus isolates

BACKGROUND: The reovirus M1 genome segment encodes the μ2 protein, a structurally minor component of the viral core, which has been identified as a transcriptase cofactor, nucleoside and RNA triphosphatase, and microtubule-binding protein. The μ2 protein is the most poorly understood of the reovirus structural proteins. Genome segment sequences have been reported for 9 of the 10 genome segments for the 3 prototypic reoviruses type 1 Lang (T1L), type 2 Jones (T2J), and type 3 Dearing (T3D), but the M1 genome segment sequences for only T1L and T3D have been previously reported. For this study, we determined the M1 nucleotide and deduced μ2 amino acid sequences for T2J, nine other reovirus field isolates, and various T3D plaque-isolated clones from different laboratories. RESULTS: Determination of the T2J M1 sequence completes the analysis of all ten genome segments of that prototype. The T2J M1 sequence contained a 1 base pair deletion in the 3' non-translated region, compared to the T1L and T3D M1 sequences. The T2J M1 gene showed ~80% nucleotide homology, and the encoded μ2 protein showed ~71% amino acid identity, with the T1L and T3D M1 and μ2 sequences, respectively, making the T2J M1 gene and μ2 proteins amongst the most divergent of all reovirus genes and proteins. Comparisons of these newly determined M1 and μ2 sequences with newly determined M1 and μ2 sequences from nine additional field isolates and a variety of laboratory T3D clones identified conserved features and/or regions that provide clues about μ2 structure and function. CONCLUSIONS: The findings suggest a model for the domain organization of μ2 and provide further evidence for a role of μ2 in viral RNA synthesis. The new sequences were also used to explore the basis for M1/μ2-determined differences in the morphology of viral factories in infected cells. The findings confirm the key role of Ser/Pro208 as a prevalent determinant of differences in factory morphology among reovirus isolates and trace the divergence of this residue and its associated phenotype among the different laboratory-specific clones of type 3 Dearing

Human monoclonal antibodies directed against toxins A and B prevent Clostridium difficile-induced mortality in hamsters

Clostridium difficile is the leading cause of nosocomial antibiotic-associated diarrhea, and recent outbreaks of strains with increased virulence underscore the importance of identifying novel approaches to treat and prevent relapse of Clostridium difficile-associated diarrhea (CDAD). CDAD pathology is induced by two exotoxins, toxin A and toxin B, which have been shown to be cytotoxic and, in the case of toxin A, enterotoxic. In this report we describe fully human monoclonal antibodies (HuMAbs) that neutralize these toxins and prevent disease in hamsters. Transgenic mice carrying human immunoglobulin genes were used to isolate HuMAbs that neutralize the cytotoxic effects of either toxin A or toxin B in cell-based in vitro neutralization assays. Three anti-toxin A HuMAbs (3H2, CDA1, and 1B11) could all inhibit the enterotoxicity of toxin A in mouse intestinal loops and the in vivo toxicity in a systemic mouse model. Four anti-toxin B HuMAbs (MDX-1388, 103-174, 1G10, and 2A11) could neutralize cytotoxicity in vitro, although systemic toxicity in the mouse could not be neutralized. Anti-toxin A HuMAb CDA1 and anti-toxin B HuMAb MDX-1388 were tested in the well-established hamster model of C. difficile disease. CDA1 alone resulted in a statistically significant reduction of mortality in hamsters; however, the combination treatment offered enhanced protection. Compared to controls, combination therapy reduced mortality from 100% to 45% (P\u3c0.0001) in the primary disease hamster model and from 78% to 32% (P\u3c0.0001) in the less stringent relapse model

Human Monoclonal Antibody HCV1 Effectively Prevents and Treats HCV Infection in Chimpanzees

Hepatitis C virus (HCV) infection is a leading cause of liver transplantation and there is an urgent need to develop therapies to reduce rates of HCV infection of transplanted livers. Approved therapeutics for HCV are poorly tolerated and are of limited efficacy in this patient population. Human monoclonal antibody HCV1 recognizes a highly-conserved linear epitope of the HCV E2 envelope glycoprotein (amino acids 412-423) and neutralizes a broad range of HCV genotypes. In a chimpanzee model, a single dose of 250 mg/kg HCV1 delivered 30 minutes prior to infusion with genotype 1a H77 HCV provided complete protection from HCV infection, whereas a dose of 50 mg/kg HCV1 did not protect. In addition, an acutely-infected chimpanzee given 250 mg/kg HCV1 42 days following exposure to virus had a rapid reduction in viral load to below the limit of detection before rebounding 14 days later. The emergent virus displayed an E2 mutation (N415K/D) conferring resistance to HCV1 neutralization. Finally, three chronically HCV-infected chimpanzees were treated with a single dose of 40 mg/kg HCV1 and viral load was reduced to below the limit of detection for 21 days in one chimpanzee with rebounding virus displaying a resistance mutation (N417S). The other two chimpanzees had 0.5-1.0 log(10) reductions in viral load without evidence of viral resistance to HCV1. In vitro testing using HCV pseudovirus (HCVpp) demonstrated that the sera from the poorly-responding chimpanzees inhibited the ability of HCV1 to neutralize HCVpp. Measurement of antibody responses in the chronically-infected chimpanzees implicated endogenous antibody to E2 and interference with HCV1 neutralization although other factors may also be responsible. These data suggest that human monoclonal antibody HCV1 may be an effective therapeutic for the prevention of graft infection in HCV-infected patients undergoing liver transplantation

Reovirus Core Protein μ2 Determines the Filamentous Morphology of Viral Inclusion Bodies by Interacting with and Stabilizing Microtubules

Cells infected with mammalian reoviruses often contain large perinuclear inclusion bodies, or “factories,” where viral replication and assembly are thought to occur. Here, we report a viral strain difference in the morphology of these inclusions: filamentous inclusions formed in cells infected with reovirus type 1 Lang (T1L), whereas globular inclusions formed in cells infected with our laboratory's isolate of reovirus type 3 Dearing (T3D). Examination by immunofluorescence microscopy revealed the filamentous inclusions to be colinear with microtubules (MTs). The filamentous distribution was dependent on an intact MT network, as depolymerization of MTs early after infection caused globular inclusions to form. The inclusion phenotypes of T1L × T3D reassortant viruses identified the viral M1 genome segment as the primary genetic determinant of the strain difference in inclusion morphology. Filamentous inclusions were seen with 21 of 22 other reovirus strains, including an isolate of T3D obtained from another laboratory. When the μ2 proteins derived from T1L and the other laboratory's T3D isolate were expressed after transfection of their cloned M1 genes, they associated with filamentous structures that colocalized with MTs, whereas the μ2 protein derived from our laboratory's T3D isolate did not. MTs were stabilized in cells infected with the viruses that induced filamentous inclusions and after transfection with the M1 genes derived from those viruses. Evidence for MT stabilization included bundling and hyperacetylation of α-tubulin, changes characteristically seen when MT-associated proteins (MAPs) are overexpressed. Sequencing of the M1 segments from the different T1L and T3D isolates revealed that a single-amino-acid difference at position 208 correlated with the inclusion morphology. Two mutant forms of μ2 with the changes Pro-208 to Ser in a background of T1L μ2 and Ser-208 to Pro in a background of T3D μ2 had MT association phenotypes opposite to those of the respective wild-type proteins. We conclude that the μ2 protein of most reovirus strains is a viral MAP and that it plays a key role in the formation and structural organization of reovirus inclusion bodies

Reovirus σNS Protein Localizes to Inclusions through an Association Requiring the μNS Amino Terminus

Cells infected with mammalian reoviruses contain phase-dense inclusions, called viral factories, in which viral replication and assembly are thought to occur. The major reovirus nonstructural protein μNS forms morphologically similar phase-dense inclusions when expressed in the absence of other viral proteins, suggesting it is a primary determinant of factory formation. In this study we examined the localization of the other major reovirus nonstructural protein, σNS. Although σNS colocalized with μNS in viral factories during infection, it was distributed diffusely throughout the cell when expressed in the absence of μNS. When coexpressed with μNS, σNS was redistributed and colocalized with μNS inclusions, indicating that the two proteins associate in the absence of other viral proteins and suggesting that this association may mediate the localization of σNS to viral factories in infected cells. We have previously shown that μNS residues 1 to 40 or 41 are both necessary and sufficient for μNS association with the viral microtubule-associated protein μ2. In the present study we found that this same region of μNS is required for its association with σNS. We further dissected this region, identifying residues 1 to 13 of μNS as necessary for association with σNS, but not with μ2. Deletion of σNS residues 1 to 11, which we have previously shown to be required for RNA binding by that protein, resulted in diminished association of σNS with μNS. Furthermore, when treated with RNase, a large portion of σNS was released from μNS coimmunoprecipitates, suggesting that RNA contributes to their association. The results of this study provide further evidence that μNS plays a key role in forming the reovirus factories and recruiting other components to them

Carboxyl-Proximal Regions of Reovirus Nonstructural Protein μNS Necessary and Sufficient for Forming Factory-Like Inclusions

Mammalian orthoreoviruses are believed to replicate in distinctive, cytoplasmic inclusion bodies, commonly called viral factories or viroplasms. The viral nonstructural protein μNS has been implicated in forming the matrix of these structures, as well as in recruiting other components to them for putative roles in genome replication and particle assembly. In this study, we sought to identify the regions of μNS that are involved in forming factory-like inclusions in transfected cells in the absence of infection or other viral proteins. Sequences in the carboxyl-terminal one-third of the 721-residue μNS protein were linked to this activity. Deletion of as few as eight residues from the carboxyl terminus of μNS resulted in loss of inclusion formation, suggesting that some portion of these residues is required for the phenotype. A region spanning residues 471 to 721 of μNS was the smallest one shown to be sufficient for forming factory-like inclusions. The region from positions 471 to 721 (471-721 region) includes both of two previously predicted coiled-coil segments in μNS, suggesting that one or both of these segments may also be required for inclusion formation. Deletion of the more amino-terminal one of the two predicted coiled-coil segments from the 471-721 region resulted in loss of the phenotype, although replacement of this segment with Aequorea victoria green fluorescent protein, which is known to weakly dimerize, largely restored inclusion formation. Sequences between the two predicted coiled-coil segments were also required for forming factory-like inclusions, and mutation of either one His residue (His570) or one Cys residue (Cys572) within these sequences disrupted the phenotype. The His and Cys residues are part of a small consensus motif that is conserved across μNS homologs from avian orthoreoviruses and aquareoviruses, suggesting this motif may have a common function in these related viruses. The inclusion-forming 471-721 region of μNS was shown to provide a useful platform for the presentation of peptides for studies of protein-protein association through colocalization to factory-like inclusions in transfected cells

Reovirus Nonstructural Protein μNS Recruits Viral Core Surface Proteins and Entering Core Particles to Factory-Like Inclusions

Mammalian reoviruses are thought to assemble and replicate within cytoplasmic, nonmembranous structures called viral factories. The viral nonstructural protein μNS forms factory-like globular inclusions when expressed in the absence of other viral proteins and binds to the surfaces of the viral core particles in vitro. Given these previous observations, we hypothesized that one or more of the core surface proteins may be recruited to viral factories through specific associations with μNS. We found that all three of these proteins—λ1, λ2, and σ2—localized to factories in infected cells but were diffusely distributed through the cytoplasm and nucleus when each was separately expressed in the absence of other viral proteins. When separately coexpressed with μNS, on the other hand, each core surface protein colocalized with μNS in globular inclusions, supporting the initial hypothesis. We also found that λ1, λ2, and σ2 each localized to filamentous inclusions formed upon the coexpression of μNS and μ2, a structurally minor core protein that associates with microtubules. The first 40 residues of μNS, which are required for association with μ2 and the RNA-binding nonstructural protein σNS, were not required for association with any of the three core surface proteins. When coexpressed with μ2 in the absence of μNS, each of the core surface proteins was diffusely distributed and displayed only sporadic, weak associations with μ2 on filaments. Many of the core particles that entered the cytoplasm of cycloheximide-treated cells following entry and partial uncoating were recruited to inclusions of μNS that had been preformed in those cells, providing evidence that μNS can bind to the surfaces of cores in vivo. These findings expand a model for how viral and cellular components are recruited to the viral factories in infected cells and provide further evidence for the central but distinct roles of viral proteins μNS and μ2 in this process