The Virus

The family Astroviridae encompasses small nonenveloped viruses that infect a variety of animal species. They were originally classified among the small round structured viruses (SRSV) causing diarrhea in humans. They are round with icosahedral symmetry and 27-30 nm in diameter. They were first described by Madeley and Cosgrove[1] as a potential cause of diarrheal disease in human infants and named for the characteristic five- or six-pointed star (astron is Greek for a star), visible on the capsid surface by negative stain electron microscopy (Fig. 1). There are small surface projections consisting of 30 dimeric spikes protruding some 50 A from the virus surface. In 1981, serial passage of human astrovirus was achieved in primary human embryo kidney (HEK) cells by Lee and Kurtz,[2] but astrovirus is more conveniently cultured in CaCo-2 cells. However, trypsin must be included in the culture medium.[3] Astrovirus particles are stable at pH 3, but disassemble at pH 10.5 and are resistant to chloroform, detergents (nonionic or ionic), and lipid solvents such as ether. At 60°C astrovirus retains infectivity for 5 but not 10 min. It is stable for years at — 70°C but is disrupted by repeated freeze-thaw cycles.[4] Human astroviruses (HuAst) survived for 5-6 days when dried at 20°C in fecal material onto porous or nonporous material. Survival was significantly longer at 4°C. Its survival was equivalent to that of adenoviruses but less good than that of rotaviruses.[5]

The genome is positive sense single-stranded linear unsegmented RNA and 6.8 to 7.9 kb long.[6] It has a poly-A tail at the 3' end. During infection both genomic (6.8 kb) and subgenomic (2.4 kb) RNA are produced. The genome encodes three ORFs. At the 5' end of the genome are encoded ORF1a and ORF1b. The third ORF (OFR-2) is found at the 3' end as is the subgenomic fragment in infected cells. ORF1a and ORF1b encode nonstructural proteins whereas ORF2 encodes the capsid proteins. At the 5' end there is an untranslated region of 80-85 nucleotides depending on the astrovirus serotype. ORF1a varies in size from 2763 to 2784 nucleotides also depending upon the serotype. There is an area of overlap between ORF1a and ORF1b of between 61 nt (for HuAst serotype 3) and 73 nt (for serotypes 1 and 2). Similarly, ORF1b varies in length from 1548 to 1560 nt depending upon serotype. ORF1a encodes a putative protein (nsp1a) of 920 to 935 amino acids in length.[6] It contains a serine protease motif similar to that seen in other positive sense RNA viruses such as feline calicivirus (FCV). However, a major difference between it and FCV is the substitution of a serine for a cysteine residue at the third catalytic amino acid residue.[7] Downstream of the protease motif is a nuclear localization signal which directs ORF1a protein to the nucleus of infected cells as has been determined by immunofluorescence and immunoprecipitation.[8,9] The 920-amino acid nsp-1a is itself proteolytically cleaved into smaller proteins and in particular, cleavage around amino acid 410 appears to be mediated by the polypeptide itself.[10] ORF1b encodes a polypeptide containing 515519 amino acids and contains motifs suggestive of an RNA-dependent RNA polymerase most closely resembling those of plant viruses, bymovirus, and potyvirus.[11] There is an overlap between the end of ORF1a and the beginning of ORF1b which is highly conserved. This suggests there is a ribosomal frameshift mechanism involved in the translation of the polypeptides.

ORF2 encodes a viral structural protein varying in length from 782 to 794 amino acids depending on serotype.[6] The polypeptide is conserved for the first

Fig. 1 Negative stain electron micrograph of astrovirus in feces.

inflammation and of enterocyte death,[18] but intestinal maltase activity decreases suggesting an osmotic mechanism for diarrhea.[19]

The major determinants of astrovirus immunity are as yet unclear. Serum antibody does seem to correlate with immunity in volunteer studies,[15] and specific T cells that could recognize astrovirus antigen have been found in the adult lamina propria.[20]

There seems to be an association between HIV infection and astrovirus infection and diarrhea.[21,22] Similar associations between astrovirus infection and other forms of immunodeficiency have been described.[6]

Fig. 1 Negative stain electron micrograph of astrovirus in feces.

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