The astrovirus N-terminal nonstructural protein anchors replication complexes to the perinuclear ER membranes.

An essential aspect of positive-sense RNA virus replication is anchoring the replication complex (RC) to cellular membranes. Positive-sense RNA viruses employ diverse strategies, including co-translational membrane targeting through signal peptides and co-opting cellular membrane trafficking components. Often, N-terminal nonstructural proteins play a crucial role in linking the RC to membranes, facilitating the early association of the replication machinery. Astroviruses utilize a polyprotein strategy to synthesize nonstructural proteins, relying on subsequent processing to form replication-competent complexes. This study provides evidence for the perinuclear ER membrane association of RCs in five distinct human astrovirus strains. Using tagged recombinant classical human astrovirus 1 and neurotropic MLB2 strains, we establish that the N-terminal domain guides the ER membrane association. We identified di-arginine motifs responsible for the perinuclear ER retention and formation of functional RCs through mutational analysis of the N-terminal domain in replicon and reverse genetics systems. In addition, we demonstrate the association of key components of the astrovirus replication complex: double-stranded RNA, RNA-dependent RNA polymerase, protease, and N-terminal protein. Our findings highlight the intricate virus-ER interaction mechanism employed by astroviruses, potentially leading to the development of novel antiviral intervention strategies.

The Roles of the 5' and 3' Untranslated Regions in Human Astrovirus Replication.

Astroviruses are small nonenveloped single-stranded RNA viruses with a positive sense genome. They are known to cause gastrointestinal disease in a broad spectrum of species. Although astroviruses are distributed worldwide, a gap in knowledge of their biology and disease pathogenesis persists. Many positive-sense single-stranded RNA viruses show conserved and functionally important structures in their 5' and 3' untranslated regions (UTRs). However, not much is known about the role of the 5' and 3' UTRs in the viral replication of HAstV-1. We analyzed the UTRs of HAstV-1 for secondary RNA structures and mutated them, resulting in partial or total UTR deletion. We used a reverse genetic system to study the production of infectious viral particles and to quantify protein expression in the 5' and 3' UTR mutants, and we established an HAstV-1 replicon system containing two reporter cassettes in open reading frames 1a and 2, respectively. Our data show that 3' UTR deletions almost completely abolished viral protein expression and that 5' UTR deletions led to a reduction in infectious virus particles in infection experiments. This indicates that the presence of the UTRs is essential for the life cycle of HAstV-1 and opens avenues for further research.

Comparative Analysis of Novel Strains of Porcine Astrovirus Type 3 in the USA.

Porcine astrovirus type 3 (PoAstV3) has been previously identified as a cause of polioencephalomyelitis in swine and continues to cause disease in the US swine industry. Herein, we describe the characterization of both untranslated regions, frameshifting signal, putative genome-linked virus protein (VPg) and conserved antigenic epitopes of several novel PoAstV3 genomes. Twenty complete coding sequences (CDS) were obtained from 32 diagnostic cases originating from 11 individual farms/systems sharing a nucleotide (amino acid) percent identity of 89.74-100% (94.79-100%), 91.9-100% (96.3-100%) and 90.71-100% (93.51-100%) for ORF1a, ORF1ab and ORF2, respectively. Our results indicate that the 5'UTR of PoAstV3 is highly conserved highlighting the importance of this region in translation initiation while their 3'UTR is moderately conserved among strains, presenting alternative configurations including multiple putative protein binding sites and pseudoknots. Moreover, two predicted conserved antigenic epitopes were identified matching the 3' termini of VP27 of PoAstV3 USA strains. These epitopes may aid in the design and development of vaccine components and diagnostic assays useful to control outbreaks of PoAstV3-associated CNS disease. In conclusion, this is the first analysis predicting the structure of important regulatory motifs of neurotropic mamastroviruses, which differ from those previously described in human astroviruses.

Identification of a novel protein in porcine astrovirus that is important for virus replication.

Overlapping genes are common in some RNA viruses. It has been proposed that a potential overlapping gene is the ORFX, here termed ORF2b, which overlaps the ORF2 coding sequence in astroviruses. The aim of this study was to determine whether ORF2b is an overlapping gene that encodes a functional protein which is needed for viral replication. Sequence alignment showed that there was an ORF2b in a PAstV type 1 strain of astrovirus, PAstV1-GX1, which was embedded within the larger ORF2. The AUG codon for ORF2b is located 19 nucleotides downstream of the initiation site of ORF2 and contains 369 nucleotides and it codes for a predicted 122-amino-acid protein. A specific polyclonal antibody against the ORF2b protein was raised and used to demonstrate the expression of the new identified gene in virus-infected and pCAGGS-ORF2b-transfected cells. Analysis of purified virions revealed that the ORF2b protein was not incorporated into virus particles. Reverse genetics based on a PAstV type 1 infectious cDNA clone showed that the ORF2b protein was not essential but important for optimal virus infectivity. Knockout of the downstream potential stop codon candidate of ORF2b demonstrated that the C-terminus of the ORF2b protein can be extended by 170 amino acids, suggesting that the C-terminus of the newly identified ORF2b protein may be variable.

A hidden gene in astroviruses encodes a viroporin.

Human astroviruses are small non-enveloped viruses with positive-sense single-stranded RNA genomes. Astroviruses cause acute gastroenteritis in children worldwide and have been associated with encephalitis and meningitis in immunocompromised individuals. It is still unknown how astrovirus particles exit infected cells following replication. Through comparative genomic analysis and ribosome profiling we here identify and confirm the expression of a conserved alternative-frame ORF, encoding the protein XP. XP-knockout astroviruses are attenuated and pseudo-revert on passaging. Further investigation into the function of XP revealed plasma and trans Golgi network membrane-associated roles in virus assembly and/or release through a viroporin-like activity. XP-knockout replicons have only a minor replication defect, demonstrating the role of XP at late stages of infection. The discovery of XP advances our knowledge of these important human viruses and opens an additional direction of research into their life cycle and pathogenesis.

Construction of a reverse genetic system for porcine astrovirus.

In order to construct a full-length infectious cDNA clone of porcine astrovirus, three fragments covering the complete genome of PAstV1-GX1 strain were amplified by RT-PCR. All three PCR-amplified fragments were cloned into T-Vector pMD19 (Simple), and subsequently assembled into a full-length cDNA clone by subcloning. A silent nucleotide change creating a PstI site was engineered into the full-length cDNA clone to distinguish the rescued virus from the parental virus. Upon transfection of BHK-21 cells with the in vitro transcripts of both the original and constructed cDNAs, typical cytopathic effects were observed on PK-15 cells after serial passaging of the cell supernatant. The construction and recovery of the infectious cDNA clone of porcine astrovirus will provide a valuable experimental system to study the genome function and pathogenesis of astroviruses.

The Astrovirus Capsid: A Review.

Astroviruses are enterically transmitted viruses that cause infections in mammalian and avian species. Astroviruses are nonenveloped, icosahedral viruses comprised of a capsid protein shell and a positive-sense, single-stranded RNA genome. The capsid protein undergoes dramatic proteolytic processing both inside and outside of the host cell, resulting in a coordinated maturation process that affects cellular localization, virus structure, and infectivity. After maturation, the capsid protein controls the initial phases of virus infection, including virus attachment, endocytosis, and genome release into the host cell. The astrovirus capsid is the target of host antibodies including virus-neutralizing antibodies. The capsid protein also mediates the binding of host complement proteins and inhibits complement activation. Here, we will review our knowledge on the astrovirus capsid protein (CP), with particular attention to the recent structural, biochemical, and virological studies that have advanced our understanding of the astrovirus life cycle.

[Prokaryotic expression and identification of human astrovirus nonstructural proteins, nsP1a and nsP1a/4].

Human astrovirus (HastV) is recognized as one of the leading causes of acute viral diarrhea in infants. The HastV non-structural protein, nsPla, and C-terminal protein, nsPla/4, contain various conserved functional domains,and may play an important role in virus replication, transcription and the virus-host interactions of HastV. This study used an E. coli system to investigate the expression of nsPla and nsPla/4 proteins. Firstly,the nsPla and nsPla/4 genes of HAstV-1 were cloned into the prokaryotic expression vector,PGEX-4T-1, to build the PGEX-4T-1a and PGEX-4T-la/4 fusion protein plasmids. Then, the recombinant plasmids were transformed into Escherichia coli BL21 (DE3) and induced with isopropyl-ß-D-thiogalactopyranoside (IPTG). The optimal expression conditions of the two fusion proteins were identified and then analyzed by polyacrylamide gel electrophoresis (SDS-PAGE) and western blotting, respectively. The results showed that the pGEX-4T-la fusion protein was maximally expressed at 30 °C after 12 hours of induction with 1.0 mM IPTG. The pGEX-4T-la/4 fusion protein was maximally expressed at 20 °C after 8 hours of induction with 0.5 mM IPTG. Western blot analysis showed that the two fusion proteins specificity reacted with the anti-nsPla and anti-GST monoclonal antibodies, respectively. This study successfully obtained the HAstV non-structural protein, nsP1a, and its C-terminal protein nsP1a/4 protein using an E. coli system. This novel study lays the foundation for future research into the pathogenic mechanisms of human astrovirus and the functions of its non-structural protein.

PTB binds to the 3' untranslated region of the human astrovirus type 8: a possible role in viral replication.

The 3' untranslated region (3'UTR) of human astroviruses (HAstV) consists of two hairpin structures (helix I and II) joined by a linker harboring a conserved PTB/hnRNP1 binding site. The identification and characterization of cellular proteins that interact with the 3'UTR of HAstV-8 virus will help to uncover cellular requirements for viral functions. To this end, mobility shift assays and UV cross-linking were performed with uninfected and HAstV-8-infected cell extracts and HAstV-8 3'UTR probes. Two RNA-protein complexes (CI and CII) were recruited into the 3'UTR. Complex CII formation was compromised with cold homologous RNA, and seven proteins of 35, 40, 45, 50, 52, 57/60 and 75 kDa were cross-linked to the 3'UTR. Supermobility shift assays indicated that PTB/hnRNP1 is part of this complex, and 3'UTR-crosslinked PTB/hnRNP1 was immunoprecipitated from HAstV-8 infected cell-membrane extracts. Also, immunofluorescence analyses revealed that PTB/hnRNP1 is distributed in the nucleus and cytoplasm of uninfected cells, but it is mainly localized perinuclearly in the cytoplasm of HAstV-8 infected cells. Furthermore, the minimal 3'UTR sequences recognized by recombinant PTB are those conforming helix I, and an intact PTB/hnRNP1-binding site. Finally, small interfering RNA-mediated PTB/hnRNP1 silencing reduced synthesis viral genome and virus yield in CaCo2 cells, suggesting that PTB/hnRNP1 is required for HAstV replication. In conclusion, PTB/hnRNP1 binds to the 3'UTR HAstV-8 and is required or participates in viral replication.

Construction of high-quality Caco-2 three-frame cDNA library and its application to yeast two-hybrid for the human astrovirus protein-protein interaction.

Human epithelial colorectal adenocarcinoma (Caco-2) cells are widely used as an in vitro model of the human small intestinal mucosa. Caco-2 cells are host cells of the human astrovirus (HAstV) and other enteroviruses. High quality cDNA libraries are pertinent resources and critical tools for protein-protein interaction research, but are currently unavailable for Caco-2 cells. To construct a three-open reading frame, full length-expression cDNA library from the Caco-2 cell line for application to HAstV protein-protein interaction screening, total RNA was extracted from Caco-2 cells. The switching mechanism at the 5' end of the RNA transcript technique was used for cDNA synthesis. Double-stranded cDNA was digested by Sfi I and ligated to reconstruct a pGADT7-Sfi I three-frame vector. The ligation mixture was transformed into Escherichia coli HST08 premium electro cells by electroporation to construct the primary cDNA library. The library capacity was 1.0×10(6)clones. Gel electrophoresis results indicated that the fragments ranged from 0.5kb to 4.2kb. Randomly picked clones show that the recombination rate was 100%. The three-frame primary cDNA library plasmid mixture (5×10(5)cfu) was also transformed into E. coli HST08 premium electro cells, and all clones were harvested to amplify the cDNA library. To detect the sufficiency of the cDNA library, HAstV capsid protein as bait was screened and tested against the Caco-2 cDNA library by a yeast two-hybrid (Y2H) system. A total of 20 proteins were found to interact with the capsid protein. These results showed that a high-quality three-frame cDNA library from Caco-2 cells was successfully constructed. This library was efficient for the application to the Y2H system, and could be used for future research.

Screening effective short interfering RNA/short hairpin RNA for inhibition of human astrovirus ORF2 gene expression in cultured cells.

In this study, we have evaluated four different 21-nt duplexes of small interfering RNA (siRNA-469, siRNA-852, siRNA-1802 and siRNA-1806) that specifically target the ORF2 gene of human astrovirus (HAstV) in inhibiting HAstV capsid protein expression in transfected BHK-21 cells. Furthermore, fluorescence analysis, real-time quantitative PCR (RT-qPCR) and western blot assays showed that pGPU6/GFP/Neo-shRNA inhibits ORF2 gene expression in Caco2 cells. The results indicate that siRNA/shRNA-469 and siRNA/shRNA-1802 can interfere with capsid protein expression in cell culture, and this provides a powerful tool for the study of HAstV gene functions and the biological properties of the capsid protein.

[Construction and expression of six deletion mutants of human astrovirus C-terminal nsP1a/4 protein].

Human astrovirus (HAstV) is one of the leading causes of actue virual diarrhea in infants. HAstV-induced epithdlial cell apoptosis plays an important role in the pathogenesis of HAstV infection. Our previous study indicated that HAstV non-structural protein nsPla C-terminal protein nsPla/4 was the major apoptosis functional protein and probably contained the main apoptosis domains. In order to screen for astrovirus encoded apoptotic protien, nsPla/4 and six turncated proteins, which possessed nsPla/4 protein different function domain ,were cloned into green fluorescent protein (GFP) vector pEG-FP-N3. After 24-72 h transfection, the fusion protein expression in BHK21 cells, was analysis by fluorescence microscope and Western blot. The results indicated seven fusion proteins were observed successfully in BHK21 cell after transfected for 24 h. Western blot analysis showed that the level of fusion protein expressed in BHK21 cells was increased significantly at 72h compared to 48h in transfected cells. The successful expression of deletion mutants of nsPla/4 protein was an important foundation to gain further insights into the function of apoptosis domains of nsPla/4 protein and it would also provide research platform to further confirm the molecule pathogenic mechanism of human astrovirus.

Adaptive immunity restricts replication of novel murine astroviruses.

The mechanisms of astrovirus pathogenesis are largely unknown, in part due to a lack of a small-animal model of disease. Using shotgun sequencing and a custom analysis pipeline, we identified two novel astroviruses capable of infecting research mice, murine astrovirus (MuAstV) STL1 and STL2. Subsequent analysis revealed the presence of at least two additional viruses (MuAstV STL3 and STL4), suggestive of a diverse population of murine astroviruses in research mice. Complete genomic characterization and subsequent phylogenetic analysis showed that MuAstV STL1 to STL4 are members of the mamastrovirus genus and are likely members of a new mamastrovirus genogroup. Using Rag1(-/-) mice deficient in B and T cells, we demonstrate that adaptive immunity is required to control MuAstV infection. Furthermore, using Stat1(-/-) mice deficient in innate signaling, we demonstrate a role for the innate immune response in the control of MuAstV replication. Our results demonstrate that MuAstV STL permits the study of the mechanisms of astrovirus infection and host-pathogen interactions in a genetically manipulable small-animal model. Finally, we detected MuAstV in commercially available mice, suggesting that these viruses may be present in academic and commercial research mouse facilities, with possible implications for interpretation of data generated in current mouse models of disease.

Potent inhibition of the classical pathway of complement by a novel C1q-binding peptide derived from the human astrovirus coat protein.

Previous work from our laboratories has demonstrated that purified, recombinant human astrovirus coat protein (HAstV CP) binds C1q and mannose-binding lectin (MBL) inhibiting activation of the classical and lectin pathways of complement, respectively. Analysis of the 787 amino acid CP molecule revealed that residues 79-139 share limited sequence homology with human neutrophil defensin-1 (HNP-1), a molecule previously demonstrated to bind C1q and MBL, inhibiting activation of the classical and lectin pathways of complement, respectively. A 30 amino acid peptide derived from this region of the CP molecule competitively inhibited the binding of wild-type CP to C1q. The parent peptide and various derivatives were subsequently assayed for C1q binding, inhibition of C1 and C4 activation as well as suppression of complement activation in hemolytic assays. The parent peptide and several derivatives inhibited complement activation in these functional assays to varying degrees. One peptide derivative in particular (E23A) displayed superior inhibition of complement activation in multiple assays of classical complement pathway activation. Further analysis revealed homology to a plant defensin allowing development of a proposed structural model for E23A. Based upon these findings, we hypothesize that further rationale optimization of E23A may result in a promising therapeutic inhibitor for the treatment of inflammatory and autoimmune diseases in which dysregulated activation of the classical and lectin pathways of complement contribute to pathogenesis.

Molecular cloning, expression and first antigenic characterization of human astrovirus VP26 structural protein and a C-terminal deleted form.

The open reading frame 2 (ORF2) of human astrovirus (HAstV) encodes the structural VP26 protein that seems to be the main antigenic viral protein. However, its functional role remains unclear. Bioinformatic predictions revealed that VP29 and VP26 proteins could be involved in virus-cell interaction. In this study, we describe for the first time the cloning and expression in Escherichia coli (E. coli) of a recombinant VP26 (rVP26) protein and a VP26 C-terminal truncated form (VP26 Delta C), followed by purification by NTA-Ni(2+) agarose affinity chromatography. Protein expression and purification were evaluated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot (WB). Then, the purified proteins were evaluated for antigenic properties in enzyme linked immunosorbent assay (ELISA) using a polyclonal antibody (PAb) and a neutralizing monoclonal antibody (nMAb) named PL2, both of them directed to HAstV. The results presented herein indicate that the C-terminal end of the VP26 protein is essential to maintain the neutralizing epitope recognized by nMAb PL2 and that the N-terminus of VP26 protein may contain antigenic lineal-epitopes recognized by PAb. Thus, these recombinant proteins can be ideal tools for further antigenic, biochemical, structural and functional VP26 protein characterization, in order to evaluate its potential role in immunodiagnosis and vaccine studies.

Astrovirus increases epithelial barrier permeability independently of viral replication.

Astrovirus infection in a variety of species results in an age-dependent diarrhea; however, the means by which astroviruses cause diarrhea remain unknown. Studies of astrovirus-infected humans and turkeys have demonstrated few histological changes and little inflammation during infection, suggesting that intestinal damage or an overzealous immune response is not the primary mediator of astrovirus diarrhea. An alternative contributor to diarrhea is increased intestinal barrier permeability. Here, we demonstrate that astrovirus increases barrier permeability in a Caco-2 cell culture model system following apical infection. Increased permeability correlated with disruption of the tight-junction protein occludin and decreased the number of actin stress fibers in the absence of cell death. Additionally, permeability was increased when monolayers were treated with UV-inactivated virus or purified recombinant human astrovirus serotype 1 capsid in the form of virus-like particles. Together, these results demonstrate that astrovirus-induced permeability occurs independently of viral replication and is modulated by the capsid protein, a property apparently unique to astroviruses. Based on these data, we propose that the capsid contributes to diarrhea in vivo.

Association of the astrovirus structural protein VP90 with membranes plays a role in virus morphogenesis.

VP90, the capsid polyprotein precursor of human astrovirus Yuc8, is assembled into viral particles, and its processing at the carboxy terminus by cellular caspases, to yield VP70, has been correlated with the cell release of the virus. Here, we characterized the effect of the VP90-VP70 processing on the properties of these proteins, as well as on their intracellular distribution. VP90 was found in membrane-enriched fractions (mVP90), as well as in fractions enriched in cytosolic proteins (cVP90), while VP70 was found exclusively in the latter fractions. Upon trypsin activation, infectivity was detected in all VP90-containing fractions, confirming that both mVP90 and cVP90 are able to assemble into particles; however, the two forms of VP90 showed differential sensitivities to trypsin, especially at their carboxy termini, which in the case of mVP90 was shown to remain membrane associated after protease digestion. Structural protein oligomers were detected in purified VP70-containing viruses, as well as in membrane-enriched fractions, but they were less evident in cytosolic fractions. Ultrastructural studies of infected cells revealed different types of viral particles, some of which appeared to be associated with membranes. By immunoelectron microscopy, structural proteins were shown to form virus particles in clusters and to associate with the edges of vesicles induced during infection, which also appear to contain subviral particles inside. Nonstructural proteins and viral RNA colocalized with mVP90, but not with cVP90, suggesting that mVP90 might represent the form of the protein that is initially assembled into particles, at the sites where the virus genome is being replicated.

Identification of structural domains involved in astrovirus capsid biology.

Coat proteins of non-enveloped, icosahedral viruses must perform a variety of functions during their life cycle such as assembly of the coat protein subunits into a closed shell, specific encapsidation of the viral nucleic acid, maturation of the capsid, interaction with host receptors, and disassembly to deliver the genetic information into the newly infected cell. A thorough understanding of the multiple capsid properties at the molecular level is required in order to identify potential targets for antiviral therapy and the prevention of viral disease. The system we have chosen for study is the astrovirus, a family of icosahedral, single-stranded RNA viruses that cause disease in mammals and birds. Very little is known about what regions of the coat protein contribute to the diverse capsid functions. This review will present novel structural predictions for the coat protein sequence of different astrovirus family members. Based on these predictions, we hypothesize that the assembly and RNA packaging functions of the astrovirus coat protein constitutes an individual domain distinct from the determinants required for receptor binding and internalization. Information derived from these structural predictions will serve as an important tool in designing experiments to understand astrovirus biology.

C-terminal nsP1a protein of human astrovirus colocalizes with the endoplasmic reticulum and viral RNA.

Computational and biological approaches were undertaken to characterize the role of the human astrovirus nonstructural protein nsP1a/4, located at the C-terminal fragment of nsP1a. Computer analysis reveals sequence similarities to other nonstructural viral proteins involved in RNA replication and/or transcription and allows the identification of a glutamine- and proline-rich region, the prediction of many phosphorylation and O-glycosylation sites, and the occurrence of a KKXX-like endoplasmic reticulum retention signal. Immunoprecipitation analysis with an antibody against a synthetic peptide of the nsP1a/4 sequence detected polyprotein precursors of 160, 75, and 38 to 40 kDa as well as five smaller proteins in the range of 21 to 27 kDa. Immunofluorescence labeling showed that the nsP1a/4 protein is accumulated at the perinuclear region, in association with the endoplasmic reticulum and the viral RNA. These results suggest the involvement of nsP1a/4 protein in the RNA replication process in endoplasmic reticulum-derived intracellular membranes.

Caspases mediate processing of the capsid precursor and cell release of human astroviruses.

In this work we have shown that astrovirus infection induces apoptosis of Caco-2 cells, since fragmentation of cellular DNA, cleavage of cellular proteins which are substrate of activated caspases, and a change in the mitochondrial transmembrane potential occur upon virus infection. The human astrovirus Yuc8 polyprotein capsid precursor VP90 is initially processed to yield VP70, and we have shown that this processing is trypsin independent and occurs intracellularly through four cleavages at its carboxy-terminal region. We further showed that VP90-VP70 processing is mediated by caspases, since it was blocked by the pancaspase inhibitor benzyloxycarbonyl-Val-Ala-Asp fluoromethylketone (z-VAD-fmk), and it was promoted by the apoptosis inducer TNF-related apoptosis-inducing ligand (TRAIL). Although the cell-associated virus produced in the presence of these compounds was not affected, the release of infectious virus to the cell supernatant was drastically reduced in the presence of z-VAD-fmk and increased by TRAIL, indicating that VP90-VP70 cleavage is important for the virus particles to be released from the cell. This is the first report that describes the induction and utilization of caspase activity by a virus to promote processing of the capsid precursor and dissemination of the viral particles.