Multiplication of infectious pancreatic necrosis virus

Molecular characterization of IPNV RNA replication intermediates during the viral infective cycle

Infectious Pancreatic Necrosis Virus (IPNV) is a bisegmented, double-stranded RNA virus, which belongs to the Birnaviridae family. In the current study, we have analyzed the RNA replication intermediates (RI) purified throughout the viral replication cycle in cultured cells. Equilibrium ultracentrifugation of infected cellular lysates resulted in two major peaks of viral components. The first peak, at a buoyant density of 1.33 g/cm(3), contained assembled IPNV viral particles A and B, whereas the second peak, located at buoyant densities >1.4 g/cm(3), contained a higher molecular weight viral ribonucleoprotein complex composed of, at least, VPg/VP1 and a heterogeneous population of single- and double-stranded viral RNA species. Interestingly, analyses of these dsRNA RI indicated that they contain single-stranded segments of incompletely synthesized positive-strands of RNA. Northern blot experiments of total RNA isolated from infected cells confirmed our proposed configuration of the RNA RI, where the full-length negative-strand of RNA is used as the template for the synthesis of several 3'-truncated forms of the positive-strand of the viral RNA. Together, our results indicate that IPNV utilizes the negative-strand of RNA as template for genome replication.

Genome assembly and particle maturation of the birnavirus infectious pancreatic necrosis virus

In this study, we have analyzed the morphogenesis of the birnavirus infectious pancreatic necrosis virus throughout the infective cycle in CHSE-214 cells by using a native agarose electrophoresis system. Two types of viral particles (designated A and B) were identified, isolated, and characterized both molecularly and biologically. Together, our results are consistent with a model of morphogenesis in which the genomic double-stranded RNA is immediately assembled, after synthesis, into a large (66-nm diameter) and uninfectious particle A, where the capsid is composed of both mature and immature viral polypeptides. Upon maturation, particles A yield particles B through the proteolytic cleavage of most of the remaining viral precursors within the capsid, the compaction of the particle (60-nm diameter), and the acquisition of infectivity. These studies will provide the foundation for further analyses of birnavirus particle assembly and RNA replication.

Infectious pancreatic necrosis virus induces apoptosis due to down-regulation of survival factor MCL-1 protein expression in a fish cell line

Infectious pancreatic necrosis virus (IPNV), a member of the virus family Birnaviridae, causes an acute, contagious disease in a number of economically important fish species. CHSE-214, a Chinook salmon embryonic cell line, when infected by IPNV showed morphological and biochemical features of apoptosis, including an intense DNA laddering pattern and blebbing of the plasma membrane, followed by formation of apoptotic bodies. The Mcl-1 gene product proved to be a member of the Bcl-2 gene family, and like Bcl-2 had the capacity to promote cell viability. Here, we investigated the pattern of expression of Mcl-1 in CHSE-214 cells infected by IPNV. We found that the Mcl-1 level decreased markedly in cells undergoing apoptosis after IPNV infection. This decrease was rapid during the first 8 h postinfection and preceded cell death. Furthermore, we found that drugs including cycloheximide, genistein and EDTA either prevented the decline in Mcl-1 levels or blocked the intense DNA laddering pattern. Other drugs like serine proteinase inhibitor, 400 microg/ml aprotinin, 400 microg/ml leupeptin and 100 microg/ml tryphostin did not. The virus gene expression pattern was examined by Western blot using antivirion polyclonal antibody and was blocked during treatment with cycloheximide, genistein and EDTA but not by serine proteinase, aprotinin, leupeptin or tryphostin. Together the data showed a striking correlation between virus replication and Mcl-1 expression in CHSE-214 cells, suggesting that the virus gene expression has a possible involvement with Mcl-1 in the regulation of apoptosis in these cells.

Infectious pancreatic necrosis virus: identification of a VP3-containing ribonucleoprotein core structure and evidence for O-linked glycosylation of the capsid protein VP2

Virions of infectious pancreatic necrosis virus (IPNV) were completely disintegrated upon dialysis against salt-free buffers. Direct visualization of such preparations by electron microscopy revealed 5.0- to 6.5-nm-thick entangled filaments. By using a specific colloidal gold immunolabeling technique, these structures were shown to contain the viral protein VP3. Isolation by sucrose gradient centrifugation of the filaments, followed by serological analysis, demonstrated that the entire VP3 content of the virion was recovered together with the radiolabeled genomic material forming the unique threadlike ribonucleoprotein complexes. In a sensitive blotting assay, the outer capsid component of IPNV, i.e., the major structural protein VP2, was shown to specifically bind lectins recognizing sugar moieties of N-acetylgalactosamine, mannose, and fucose. Three established metabolic inhibitors of N-linked glycosylation did not prevent addition of sugar residues to virions, and enzymatic deglycosylation of isolated virions using N-glycosidase failed to remove sugar residues of VP2 recognized by lectins. However, gentle alkaline beta elimination clearly reduced the ability of lectins to recognize VP2. These results suggest that the glycosylation of VP2 is of the O-linked type when IPNV is propagated in RTG-2 cells.

Purification of infectious pancreatic necrosis virus by anion exchange chromatography increases the specific infectivity

An improved method for the isolation and purification of infectious pancreatic necrosis virus (IPNV) is described. Virions released into the clarified growth medium are adsorbed to an anion exchange resin of diethylaminoethyl cellulose at pH 8.1. IPNV together with the likewise released and accumulated excess pool of the precursor to the major capsid protein, ICP62, are eluted at a salt concentration between 100 and 125 mM NaCl. The bovine serum albumin content of the growth medium supplement also elutes close to this position. Upon one step of combined sucrose- and CsCl-gradient centrifugation the recovered viruses display lower levels of aggregation, higher specific nucleic acid contents and an approximately 350% higher specific infectivity as compared with pools of viruses processed in parallel and isolated according to the established method relying on precipitation with poly(ethylene glycol).

Fish cell lines: establishment and characterization of three new cell lines from grass carp (Ctenopharyngodon idella)

Three new cell lines were established from tissues of the grass carp, Ctenopharyngodon idella. Derived from the fin, snout, and swim bladder of two apparently healthy diploid fry, these cell lines have been designated GCF, GCS-2, and GCSB, respectively. The cells grew at temperatures between 24 degrees and 36 degrees C with optimal growth at 32 degrees C and have been subcultured more than 50 times since their initiation in August 1986. Two of the lines remained diploid or pseudodiploid after 38 passages. The cells were tested for microbial contamination, and plating efficiencies were determined. The three cell lines were sensitive to Rhabdovirus carpio (RVC), infectious hematopoietic necrosis virus (IHNV), golden shiner virus (GSV), chum salmon virus (CSV), and infectious pancreatic necrosis virus serotype VR299 IPNV). They were refractory to channel catfish virus (CCV), channel catfish reovirus (CRV), chinook salmon paramyxovirus (CSP), and an Ab serotype of IPNV.

The viral diseases of fish: a review through 1978. Part 1: Diseases of proven viral etiology

In this review, a survey is made of the published literature on the viral diseases of fish available up to and including the year 1978. It is divided into two main sections. Part 1 describes 11 diseases where a virus has been isolated and proven to be the causative agent. Part 2 discusses 16 diseases where there is reason to suspect viral etiology because of evidence deriving from electron microscopy or transmission experiments with bacteria-free filtrates of homogenates of diseased tissue, but where final proof of a causative relationship is lacking. The review attempts to provide the most significant information on the disease process itself, in most cases including external signs, fish species susceptible, pathology, geographic distribution, existence of carriers, methods of transmission, and control. It also gives the most recent and significant data concerning the nature of the causative virus, including its cultural, biological, and physicochemical properties, where such information is available.

Peptide map comparison of infectious pancreatic necrosis virus-specific polypeptides

An investigation of virus-specific protein synthesis in infectious pancreatic necrosis virus (IPNV)-infected rainbow trout gonad cells was undertaken to find a relationship between the coding capacity of the virus genome (two segments of double-stranded RNA of 2.5 x 10(6) and 2.3 x 10(6) molecular weight) and the sizes and relative amounts of virus-specific proteins. Using polyacrylamide slabgel electrophoresis and autoradiography, eight distinct virus-specific polypeptides were detected in infected, [(35)S]methionine-labeled cells. These proteins may be grouped into three size classes on the basis of molecular weight: (i) large, alpha (90,000); (ii) medium, beta(1) (59,000), beta(2) (58,000), and beta(3) (57,000); and (iii) small, gamma(1) (29,000), gamma(1A) (28,000), gamma(2) (27,000), and gamma(3) (25,000). The combined molecular weight of these polypetides (373,000) is beyond the coding capacity of the virus genome. Purified IPNV contained polypeptides alpha, beta(3), gamma(1), and gamma(1A). Pulse-chase experiments and tryptic peptide map comparisons revealed that only four of the eight intracellular proteins were primary gene products, namely, alpha, beta(1), gamma(1), and beta(2), with a combined molecular weight of 205,000. Of these primary gene products only the alpha polypeptide was found to be stable, whereas the other three underwent intracellular proteolytic cleavage during virus morphogenesis. Polypeptide beta(1) was cleaved to generate beta(2) and beta(3); gamma(1) was trimmed to produce gamma(1A), and the only nonstructural primary gene product, gamma(2), was found to be a precursor of gamma(3). These results suggest that IPNV possesses a unique mechanism to synthesize three size classes of proteins using mRNA transcripts from two high-molecular-weight double-stranded RNA genome segments.

Biophysical studies of infectious pancreatic necrosis virus

The molecular weight of infectious pancreatic necrosis virus (IPNV) has been determined by analytical ultracentrifugation and dynamic light scattering. The sedimentation coefficient of the virus was found to be 435S. The average value for molecular weight is (55 +/- 7) x 106. The virus genome consists of two segments of double-stranded RNA (molecular weights, 2.5 x 106 and 2.3 x 106), which represents 8.7% of the virion mass. The capsid protein moiety of IPNV consists of four species of polypeptides, as determined by polyacrylamide gel electrophoresis. The number of molecules of each polypeptide in the virion has been determined. There are 22 molecules of the internal polypeptide alpha (molecular weight, 90,000), 544 molecules of the outer capsid polypeptide beta (molecular weight, 57,000), and 550 and 122 molecules, respectively, of the internal polypeptides gamma1 (molecular weight, 29,000) and gamma2 (molecular weight, 27,000). IPNV top component contains only the beta polypeptide species, and its molecular weight is estimated to be 31 x 106. The hydrodynamic diameter and electron microscopic diameter (calculated by catalase crystal-calibrated electron microscopy) of IPNV was compared with those of reovirus and encephalomyocarditis virus. Due to the swelling of the outer capsid, reovirus particles were found to be much larger when hydrated (96-nm diameter) than when dehydrated (76-nm diameter), having a large water content content and low average density. In contrast, IPNV particles are more rigid, having nearly the same average diameter under hydrous (64 nm) as under anhydrous conditions (59.3 nm). Encephalomyocarditis virus has a very low water content and does not shrink at all when prepared for electron microscopy.

Virus-specific protein synthesis in cells infected by infectious pancreatic necrosis virus

A study of virus-specific protein synthesis in infectious pancreatic necrosis virus-infected RTG-2 cells was undertaken to find a relationship between the coding capacity of virus genome (two segments of double-stranded RNA of 2.5 X10(6) and 2.3 X 10(6) molecular weight) and the sizes and relative amounts of polypeptides in the virion and in infected cells. The time course of virus-specific protein synthesis was followed by pulse labeling infected UV-irradiated cells with [35S]methionine and analyzing the labeled proteins by polyacrylamide gel electrophoresis followed by autoradiography. Three size classes of virus-specific polypeptides were synthesized, in the same relative proportion, throughout the infectious cycle, beginning 3 h postinfection. Their designation and molecular weight was as follows: alpha1, 1000,000; alpha2, 90,000; beta1, 59,000; beta2, 56,000; gamma1, 32,000; gamma2, 30,000; and gamma3, 28,000. Experiments using amino acid analogues, protease inhibitors, ZnCl2, and supraoptimal temperatures showed that polypeptides of the beta and gamma families did not arise from the alpha polypeptides by post-translational cleavage. Slow cleavage late in the infectious cycle could be demonstrated, since during 12-h period radioactivity was chased from beta1 via beta3 to beta4 (molecular weight 50,000) and beta5 (molecular weight, 49,000). During the chase most of gamma2 was degraded, whereas radioactivity could not be chased from the remaining virus-specific polypeptides. Purified virus contained polypeptides alpha1, alpha2, beta4, beta5, and gamma1. The beta polypeptides made up over 60% of the virion proteins. The results suggest that infectious pancreatic necrosis vibrus possesses a unique mechanism for synthesis of three size-classes of proteins using mRNA transcripts from two high-molecular-weight double-stranded RNA genome segments.

Size and structure of the genome of infectious pancreatic necrosis virus

The genome of infectious pancreatic necrosis virus consists of two segments of dsRNA, in equimolar amounts, with molecular weights of 2.5 X 10(6) and 2.3 X 10(6) daltons, as determined by polyacrylamide gel electrophoresis and autoradiography. The viral RNA was resistant to ribonuclease, and in sucrose gradient it co-sedimented at 14S with RNase resistant RNA from virus infected cells. Upon denaturation in 98% formamide, the viral genome sedi-mented at 24S in formamide sucrose gradient and became sensitive to RNase. Denatured 24S viral RNA did revert to its undenatured 14S form upon recentrifugation in aquaeous sucrose gradient (0.1 M NaCL), but co-sedimented with the denatured large size class of reovirus 25S RNA. The same results were obtained if the native viral RNA was pre-treated with ribonuclease before denaturation, indicating the absence of exposed single strainded regions in the viral genome. Since infectious pancreatic necrosis virus contains only two dsRNA segments it does not belong to the family Reoviridae and may represent a new group of viruses.

Effect of temperature on survival and growth of infectious pancreatic necrosis virus

Infectious pancreatic necrosis virus was stable for 10 days at 4 C in stream and well water, after which the virus had a half-life of 7.5 days. At 15 C, the virus was stable for 5 days, and then had a half-life between 5 and 6 days. Viral antigen in infected cells developed much more slowly at 4 C than at 20 C. Infected cells released infectious viral particles at temperatures as low as 4 C. Nutrition had a greater effect on the production of infectious virus at 4 C than at 20 C.

Electron microscopical and biochemical characterization of infectious pancreatic necrosis virus

An electron microscopical and biochemical examination of the properties of infectious pancreatic necrosis virus (IPN) and of its ribonucleic acid (RNA) was made. The buoyant density of IPN in CsCl was found to be 1.33 g/cm(3). Electron microscopical examination of the banded virus revealed structures similar in size (74 nm) and shape to reoviruses but lacking a characteristic inner capsid structure. Polyacrylamide gel electrophoretic analysis of IPN-RNA revealed a single non-segmented component of molecular weight 3.2 x 10(6). Its susceptibility to ribonuclease, base composition, and resistance to thermal denaturation indicated a single-stranded RNA structure. However, its sedimentation behavior (16S) independent of ionic strength in sucrose gradients, partial solubility in 2 m LiCl, and ribonuclease resistance in the presence of Mg(2+) suggest an unusual secondary structure of unknown nature. The accumulated data indicate that IPN virus does not belong to either the picornavirus or reovirus groups and may represent a new group of viruses.

Ultrastructure and sequential development of infectious pancreatic necrosis virus

The morphology and sequential development of infectious pancreatic necrosis (IPN) virus, a pathogen of trouts, were studied by electron microscopy. Mature virions were seen in the cytoplasm of infected cells incubated at 24 C as early as 6 hr after infection. These virions were hexagonal in profile and approximately 55 nm in diameter. Generally between 8 to 10 hr after infection, virus crystals of various sizes were occasionally observed. Although virus replication did not appear to be confined to a particular cytoplasmic locus, mature virions were sometimes seen in association with unidentified tubular structures approximately 45 nm in outside diameter. Negative stains of virus revealed unenveloped icosahedra approximately 65 nm in diameter with probably 92 capsomeres. Contrary to a previous communication which reported IPN virus to have picornavirus-like morphology, we found it to morphologically resemble members of the reovirus group.