Genetic characterization of feline calicivirus strains associated with varying disease manifestations during an outbreak season in Missouri (1995-1996).

Feline calicivirus (FCV) is a common cause of mild to severe upper respiratory tract disease (URTD) in cats. FCV strain 21223 was isolated from a kitten with severe pneumonia in a disease outbreak with unusually high mortality (35 %) that occurred in a Missouri feline colony in 1995-1996. Phylogenetic analysis of the genome sequence of strain 21223 indicated the emergence of a new FCV strain. Analysis of the full-length genome sequence of a closely related (99.5 % nucleotide identity) strain, 3786, obtained from an asymptomatic animal in the same colony four months later, showed the presence of seven amino acid substitutions, with six of them located in the VP1 capsid sequence encoded by ORF2. Comparative analysis of the E-region sequences (426-521 aa ORF2) presumably involved in virus-host cell receptor interactions did not identify amino acid substitutions unique to the virulent strain. We determined the complete genome sequences of four virus isolates that were collected in regional catteries in the months following the outbreak that were associated with different manifestations of the disease (URTD, chronic stomatitis, and gingivitis). We show that genetically distinct FCV strains were cocirculating in the area, and no apparent correlation could be made between overall sequence and observed disease.

Apoptosis in murine norovirus-infected RAW264.7 cells is associated with downregulation of survivin.

Noroviruses (NVs) are recognized as a major cause of nonbacterial gastroenteritis in humans. Studies of the human NVs continue to be hampered by the inability to propagate them in any cell culture system. Until recently, most data concerning NV replication were derived from studies of feline calicivirus and rabbit hemorrhagic disease virus, which are cultivable members of the family Caliciviridae. From such studies, it was proposed that caliciviruses induce apoptosis to facilitate the dissemination of viral progeny in the host. The discovery that MNV type 1 (MNV-1) grows in RAW264.7 cells provided the first cell culture system for use in studying the role of apoptosis in NV infection. We first showed that MNV-1 replication triggered apoptosis in infected RAW264.7 cells and then demonstrated that cell death was associated with activation of caspase-9 and caspase-3 through the mitochondrial pathway. This process was dependent on virus replication, since inactivated virus failed to induce signs of apoptosis. In order to better understand the apoptotic process induced by MNV-1 infection of RAW264.7 cells, we investigated the expression profiles of MNV-1-infected versus mock-infected cells. Survivin, a member of the inhibitor of apoptosis protein family, was found to be significantly downregulated in an inverse relationship with the virus genome replication. This study showed that, unlike other viruses that upregulate survivin, MNV-1 is the first virus found to downregulate the levels of survivin. We observed that MNV-1 replication in RAW264.7 cells activated caspases, resulting in apoptosis through the mitochondrial pathway, possibly as a result of downregulation of survivin.

Cleavage map and proteolytic processing of the murine norovirus nonstructural polyprotein in infected cells.

Murine norovirus (MNV) is presently the only member of the genus Norovirus in the Caliciviridae that can be propagated in cell culture. The goal of this study was to elucidate the proteolytic processing strategy of MNV during an authentic replication cycle in cells. A proteolytic cleavage map of the ORF1 polyprotein was generated, and the virus-encoded 3C-like (3CL) proteinase (Pro) mediated cleavage at five dipeptide cleavage sites, 341E/G342, Q705/N706, 870E/G871, 994E/A995, and 1177Q/G1178, that defined the borders of six proteins with the gene order p38.3 (Nterm)-p39.6 (NTPase)-p18.6-p14.3 (VPg)-p19.2 (Pro)-p57.5 (Pol). Bacterially expressed MNV 3CL Pro was sufficient to mediate trans cleavage of the ORF1 polyprotein containing the mutagenized Pro sequence into products identical to those observed during cotranslational processing of the authentic ORF1 polyprotein in vitro and to those observed in MNV-infected cells. Immunoprecipitation and Western blot analysis of proteins produced in virus-infected cells demonstrated efficient cleavage of the proteinase-polymerase precursor. Evidence for additional processing of the Nterm protein in MNV-infected cells by caspase 3 was obtained, and Nterm sequences 118DRPD121 and 128DAMD131 were mapped as caspase 3 cleavage sites by site-directed mutagenesis. The availability of the MNV nonstructural polyprotein cleavage map in concert with a permissive cell culture system should facilitate studies of norovirus replication.