Article Figures & Data
Figures
- FIG 1
Human astrovirus particles observed by transmission immunoelectron microscopy in feces negatively stained with phosphotungstic acid. Bar, 50 nm.
- FIG 2
Genome organization and polyprotein products of human astrovirus. (A) Genomic and subgenomic RNA organization, with open reading frames (ORFs) ORF1a, ORF1b, ORF2, and putative ORFX represented as boxes. Nucleotide sequences represent highly conserved sequences located in the ribosomal frameshift (RFS) signal and upstream of the initiation site of subgenomic RNA transcription. Putative RNA secondary structures conserved in the RFS and in the 3′ end of the genome are depicted. (B) Characteristic motifs of the HAstV polyprotein products. HEL, putative helicase domain; TM, transmembrane domain; CC, coiled-coil domain; PRO, protease domain; VPg, coding region for a VPg protein; HVR, hypervariable region; NLS, putative nuclear localization signal; DD, putative death domain; RdRp, RNA dependent-RNA polymerase motif. The ORF2-encoded structural polyprotein (VP90) consists of conserved regions (shell and P1 domains), a variable region containing the P2 domain (capsid spikes), and an acidic C-terminal region which is cleaved by cellular caspases to result in the VP70 precursor. Particles containing VP70 are further cleaved by trypsin to yield the VP34, VP27/29, and VP25/26 proteins.
- FIG 3
Replication of human astroviruses. After binding to one or more cellular receptors, virus uptake occurs via clathrin-dependent endocytosis. A drop in the pH leads to viral uncoating. Two main nonstructural polyproteins, nsP1a and nsP1a1b, are translated from the VPg-linked genomic RNA and further cleaved by viral and cellular proteases, resulting in mature nonstructural proteins (nsPs), which are required for genome replication. Replication complexes assemble in close association with intracellular membranes. The nsP1a/4 protein or one of its precursors, which would include the VPg domain, may interact with the RdRp protein and contribute to the regulation of the synthesis of negative- and positive-sense RNA strands as well as of subgenomic RNA. The phosphorylation status of the nsP1a/4 protein may contribute to this regulation step. Subgenomic RNAs are produced in large quantities and are used for the expression of capsid proteins. The structural VP90 polyprotein initially assembles into immature virions in association with intracellular membranes. Several cellular caspases further cleave these VP90 polyproteins once they have dissociated from membranes, resulting in VP70 immature viral capsids. Release of VP70 particles into the medium seems to occur without cell lysis, and virions mature extracellularly by the action of trypsin.
- FIG 4
Putative processing of the astrovirus nonstructural proteins expressed from ORF1a and ORF1b. HEL, putative helicase domain; CC, coiled-coil domain; TM, transmembrane domain; PRO, protease domain; VPg, viral protein genome-linked coding region; HVR, hypervariable region; RdRp, RNA-dependent RNA polymerase motif; P, posttranslational phosphorylation. Triangles depict proteolytic cleavage sites described in the literature (75, 77, 78, 80).
- FIG 5
HAstV particles are associated with membranes in infected cells. Ultrastructural analysis of CaCo-2 cells infected with HAstV-4 at 48 h postinfection shows aggregates of virions accumulated in the cytoplasm in close association with double-membrane vesicles (V). Nuclei (N) of infected cells show masses of condensed chromatin dispersed at the periphery (c).
- FIG 6
Phylogenetic relationships within the family Astroviridae. The predicted amino acid sequences of the entire capsid polyprotein were aligned using Clustal Omega (309). The phylogenetic tree was generated using the neighbor-joining algorithm (310) implemented in the MEGA6 program (311). The percentages of replicate trees in which the associated taxa clustered together in the bootstrap test (1,000 replicates) which are higher than 70 are shown next to the branches. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances (p-dist) used to infer the phylogenetic tree. All positions containing alignment gaps and missing data were removed only in pairwise sequence comparisons (pairwise deletion option). Genotype species officially recognized by the ICTV are indicated, with hosts of origin shown in parentheses. Human viruses are colored in red, porcine viruses are colored in dark blue, mink viruses are colored in dark gray, bovine viruses are colored in brown, ovine viruses are colored in light blue, bat viruses are colored in black, murine viruses are colored in purple, turkey viruses are colored in orange, and chicken viruses are colored in green. Arrows indicate strains which have been isolated from neurologic tissues. HAstV, human astrovirus; FAstV, feline astrovirus; PAstV, porcine astrovirus; CaAstV, canine astrovirus; RabAstV, rabbit astrovirus; OAstV, ovine astrovirus; BoAstV, bovine astrovirus; MuAstV, murine astrovirus; WBAstV, wild boar astrovirus; CcAstV, deer astrovirus; TAstV, turkey astrovirus; CAstV, chicken astrovirus; ANV, avian nephritis virus; WPIAstV, wild pigeon astrovirus.
- FIG 7
Flowchart of potential routes of environmental transmission of human astroviruses (see the text for details).
Tables
- TABLE 1
Main characteristics of the Astroviridae family
Characteristic Virion structure Icosahedral particles, 28–41 nm in diam Nonenveloped Distinctive five- or six-pointed star-like shape under the electron microscope in about 10% of the virions Genome A single molecule of infectious, positive-sense ssRNA, 6.4–7.7 kb, with a poly(A) tail Three ORFs: ORF1a and ORF1b at the 5′ end encoding the nonstructural proteins and ORF2 at the 3′ end encoding the structural proteins VPg protein linked to the 5′ end A ribosomal frameshifting signal present between ORF1a and ORF1b Lack of a helicase domain Replicative cycle Structural proteins are expressed from an ∼2.8-kb subgenomic RNA - TABLE 2
Studies of human astrovirus prevalence in stool
Region and country (setting, period of study) Age (yr) of study population Astrovirus prevalence (%) Detection method Reference North Africa Egypt (rural, 1995–1998) <2 26.7 EIA 237 Tunisia (urban and rural 2003–2005) <5 6.0 EIA 306 Sub-Saharan Africa Botswana (urban, 2001–2002) <5 2.7 EIA 245 Kenya (urban and rural 1999–2005) <5 5.3 EIA 246 Ghana (urban, 2005–2006) <12 4.8 RT-PCR 247 Asia Thailand (urban, 1985–1987) <5 8.6 EIA 239 Japan (urban and rural 1982–1992) Children of all age groups, mostly <10 18 EIA 249 China (urban and rural, 1998–2005) <5 5.5 EIA plus RT-PCR 307 Vietnam (urban, 2005–2006) <3 13.9 RT-PCR 241 India (urban, 2004–2008) <8 years, adults 3.1 RT-PCR 308 Japan (urban 2008–2009) Children of all age groups 1.7 RT-PCR 250 North America Mexico (rural, 1992–1995) <3 61 EIA and RT-PCR 94 USA (urban, 1993–1994) <3 20 EIA 266 USA (urban, 1996–1997) Children of all age groups, mostly <3 6.9 EIA 230 Central America Guatemala (rural, 1987–1989) <3 38.6 EIA 33 South America Chile (urban, 1985–1987; 1993–1995) <3 23.5 EIA 240 Brazil (urban, 1990–1992) <2 5.8 EIA plus RT-PCR 265 Argentina (urban, 1995–1998) <3 3.7 EIA plus RT-PCR 267 Colombia and Venezuela (urban, 1997–1999) <5 5.0 EIA 167 Europe France (urban, 1995–1998) <3 6.0 EIA 242 Spain (urban, 1997–2000) Children of all age groups, Mostly <5 4.9 RT-PCR 164 Italy (urban, 1999–2000) <2 3.1 EIA 264 Germany (urban, 2000) All ages 1.2 EIA 276 France (urban, 2007) <5 1.8 EIA 248 Oceania Australia (urban, 1995) <5 4.2 Northern (RNA) dot blot 35 - TABLE 3
Prevalence of serotypes (genotypes) of classic human astrovirus
Region and country (setting, period of study) Prevalence (%) of serotype: Reference 1 2 3 4 5 6 7 8 North Africa Egypt (rural, 1995–1998) 43.3 3.6 12.0 4.8 15.7 7.2 0 12.0 237 Asia Vietnam (urban, 2005–2006) 100.0 0 0 0 0 0 0 0 241 India (urban, 2004–2008) 6707 9.7 0 0 6.5 a 0 0 16.0 308 Japan (urban 2008–2009) 91.0 0 9.0 0 0 0 0 0 250 North America USA (urban, 1993–1994) 55.0 17.0 — b — — — — — 266 South America Brazil (urban, 1990–1992) 45.5 27.3 12.1 12.1 0 3.0 0 0 265 Argentina (urban, 1995–1998) 41.0 13.0 13.0 25.0 8.0 0 0 0 267 Europe Spain (urban, 1997–2000) 38.0 6.0 19.0 26.0 0 0 0 11.0 164 Oceania Australia (urban, 1995) 85.0 0 0 15.0 0 0 0 0 35
