Growth of the parvovirus minute virus of mice MVMp3 in EL4 lymphocytes is restricted after cell entry and before viral DNA amplification: cell-specific differences in virus uncoating in vitro.

Two murine parvoviruses with genomic sequences differing only in 33 nucleotides (8 amino acids) in the region coding for the capsid proteins show different host cell specificities: MVMi grows in EL4 T lymphocytes and MVMp3 grows in A9 fibroblasts. In this study we compared the courses of infections with these two viruses in EL4 cells in order to investigate at which step(s) the infection process of MVMp3 is interrupted. The two viruses bound equally well to EL4 cells, and similar amounts of MVMi and MVMp3 input virion DNA appeared in the nuclear fractions of EL4 cells 1 h after infection. However, double-stranded replicative-form (RF) DNA of the two viruses appeared at different times, at 10 h postinfection with MVMi and at 24 h postinfection with MVMp3. The amount of MVMp3 RF DNA detected at 24 h was very small because it was produced only in a tiny subset of the population of EL4 cells that proved to be permissive for MVMp3. Replication of double-stranded viral DNA in EL4 cells was measured after transfection of purified RF DNA, cloned viral DNA, and cloned viral DNA with a mutation preventing synthesis of the capsid proteins. In each of these cases, DNA replication was comparable for MVMi and MVMp3. Production of virus particles also appeared to be similar after transfection of the two types of RF DNA into EL4 cells. Conversion of incoming 32P-labeled single-stranded MVM DNA to 32P-labeled double-stranded RF DNA was detected only after RF DNA amplification, indicating that few molecules serve as templates for viral DNA amplification. We showed that extracts of EL4 cells contain a factor which can destabilize MVMi virions but not MVMp3 by testing the sensitivity of viral DNA to DNase and by CsCl gradient analyses of viral particles. We therefore conclude that the MVMp3 life cycle is arrested after the transport of virions to the nucleus and prior to the replication of RF DNA, most likely at the stage of viral decapsidation.     

The capsid determinant of fibrotropism for the MVMp strain of minute virus of mice functions via VP2 and not VP1.

The minute virus of mice, prototype strain MVMp, productively infects cultured murine fibroblasts but not T cells. The immunosuppressive strain, MVMi, shows the converse tropism. These reciprocal tropisms are mediated by the viral capsids, in which their determinants have been mapped to a few specific amino acids in the primary sequence shared by VP1 and VP2. Which of these proteins is relevant in presenting these determinants during infection is not known. We have approached this question using a recombinant parvovirus system in which a LuIII-derived transducing genome, containing the luciferase reporter in place of viral coding sequences, can be packaged by capsid proteins from separate helper sources. We generated transducing virions by using helper constructs expressing either VP1 or VP2, containing the MVMp or MVMi tropic determinant region, in various combinations. The virions were used to infect human NB324K cells and murine A9 fibroblasts. Transduction of the human cells (permissive for both MVMp and MVMi) required both VP1 and VP2 and was successful with all combinations of these proteins. In contrast, significant transducing activity for A9 cells was detected only with recombinant virions containing VP2 of MVMp, while the use of either source of VP1 had little effect. We conclude that VP2 from MVMp is necessary to enable infection of murine A9 fibroblasts.     

Characterization of an immunosuppressive parvovirus related to the minute virus of mice.

We have characterized an immunosuppressive parvovirus related to the minute virus of mice (MVM). The parvovirus, MVM(i), grew efficiently on the murine lymphoma cell line EL-4 and not on the A-9 strain of L-cells which is a host for the prototype MVM. MVM(i) was immunosuppressive for allogeneic mixed leukocyte cultures, inhibiting the generation of cytolytic T lymphocytes. MVM had no effect on mixed leukocyte cultures. MVM and MVM(i) particles were similar in buoyant density, sedimentation rate, appearance in the electron microscope, and polypeptide composition. We present restriction enzyme maps of the DNAs of MVM and MVM(i) which show that they are closely related. Out of 109 restriction endonuclease cleavage sites (representing together about 10% of the nucleotide sequence), 86 sites were shared by MVM and MVM(i), whereas 22 sites were absent from one of the two viruses. MVM(i) DNA had an apparent deletion of about 60 nucleotides relative to MVM, located near the 5' terminus of viral DNA.     

Myeloid depression follows infection of susceptible newborn mice with the parvovirus minute virus of mice (strain i).

The in vivo myelosuppressive capacity of strain i of the parovirus minute virus of mice (MVMi) was investigated in newborn BALB/c mice inoculated with a lethal intranasal dose. MVMi infection reached maximum levels of DNA synthesis and infectious titers in lymphohemopoietic organs at 4 to 6 days postinoculation and was restricted by an early neutralizing humoral immune response. After viral control (by 10 days postinoculation), a significant decrease in femoral and splenic cellularity, as well as in granulocyte-macrophage colony-forming unit and erythroid burst-forming unit hemopoietic progenitors, was observed in most inoculated animals. This delayed myeloid depression, although it may be not a major cause of the lethality of the infection, implies indirect pathogenic mechanisms induced by MVMi infection in a susceptible host.     

CRM1 mediates nuclear export of nonstructural protein 2 from parvovirus minute virus of mice.

The nonstructural protein 2 (NS2) from parvovirus minute virus of mice (MVMp) is a 25-kDa polypeptide which localizes preferentially to the cytoplasm and associates with cellular proteins in cytoplasm. These lines of evidence suggest that NS2 is positively exported from the nucleus to cytoplasm and functions in cytoplasm. We report here that nuclear export of NS2 is inhibited by leptomycin B (LMB), a drug that specifically blocks nuclear export signal (NES)-chromosomal region maintenance 1 (CRM1) interactions. CRM1 binds specifically to the 81- to 106-amino-acid (aa) region of NS2, and the region of NS2 actually functions as a NES. Interestingly, this region appears to be distinct from a typical NES sequence, which consists of leucine-rich sequences. These results indicate that NS2 protein is continuously exported from the nucleus by a CRM1-dependent mechanism and suggest that CRM1 also exports to distinct type of NESs.     

Construction of an infectious molecular clone of the autonomous parvovirus minute virus of mice.

The linear single-stranded DNA genome of minute virus of mice, an autonomous parvovirus, was cloned in duplex form into the bacterial plasmid pBR322. The recombinant clones of minute virus of mice were infectious when transfected into monolayers of human 324K cells and produced virus plaques with an efficiency of about 6% that obtained with duplex replicative-form DNA purified from cells infected with minute virus of mice. Southern blot analysis of transfected cells indicated that the cloned minute virus of mice genome requires both termini to be intact for excision and replication as a linear duplex molecule.     

Intracellular DNA of the parvovirus minute virus of mice is organized in a minichromosome structure.

Minute virus of mice (MVM) nucleoprotein complexes were leached from infected cell nuclei in the presence of a hypotonic buffer. Detailed biochemical analyses performed on the extracted complexes revealed nucleoprotein complexes sedimenting together with virions at 110S and defective particles sedimenting at 50S. In contrast to the virions, the nucleoprotein complexes were found to be sensitive to treatment with DNase, Sarkosyl, and heparin. They were found to be composed of replicative forms of MVM DNA and cellular histones. After extensive micrococcal nuclease digestion performed on purified nucleoprotein complexes, a viral nucleosomes core containing a DNA segment of about 140 base pairs in length was identified. These complexes when visualized by electron microscopy revealed the existence of beaded structures (minichromosomes) having 26 and 52 beads per monomer and dimer molecules, respectively. We suggest that the organization of the intracellular viral DNA in a minichromosome structure is an essential step in the virus growth cycle.     

A beta-stranded motif drives capsid protein oligomers of the parvovirus minute virus of mice into the nucleus for viral assembly

The determinants of nuclear import in the VP-1 and VP-2 capsid proteins of the parvovirus minute virus of mice strain i (MVMi) synthesized in human fibroblasts were sought by genetic analysis in an infectious plasmid. Immunofluorescence of transfected cells revealed that the two proteins were involved in cooperative cytoplasmic interactions for nuclear cotransport. However, while VP-1 translocated regardless of extension of deletions and did not form capsid epitopes by itself, VP-2 seemed to require cytoplasmic folding and the overall conformation for nuclear transport. The sequence (528)KGKLTMRAKLR(538) was found necessary for nuclear uptake of VP-2, even though it was not sufficient to confer a nuclear localization capacity on a heterologous protein. In the icosahaedral MVMi capsid, this sequence forms the carboxy end of the amphipathic beta-strand I (betaI), and all its basic residues are contiguously positioned at the face that in the unassembled subunit would be exposed to solvent. Mutations in singly expressed VP-2 that either decrease the net basic charge of the exposed face (K530N-R534T), perturb the hydrophobicity of the opposite face (L531E), or distort the betaI conformation (G529P) produced cytoplasmic subviral oligomers. Particle formation by betaI mutants indicated that the basic residues clustered at one face of betaI drive VP oligomers into the nucleus preceding and uncoupled to assembly and that the nuclear environment is required for MVMi capsid formation in the infected cell. The degree of VP-1/VP-2 transport cooperativity suggests that VP trimers are the morphogenetic intermediates translocating through the nuclear pore. The results support a model in which nuclear transport signaling preserves the VP-1/VP-2 stoichiometry necessary for efficient intranuclear assembly and in which the beta-stranded VP-2 nuclear localization motif contributes to the quality control of viral morphogenesis.     

Mapping of the fibrotropic and lymphotropic host range determinants of the parvovirus minute virus of mice.

The fibrotropic and lymphotropic strains of minute virus of mice are each unable to grow lytically in the differentiated host cell type of the other strain. To map the viral sequence responsible for the target cell specificities of the two strains, we constructed chimeric viral genomes in vitro from infectious genomic clones. The phenotypes of viral progeny derived from the chimeric genomes were tested by transfecting the plasmids into fibroblast monolayers and assaying plaque formation and by testing stocks of the recombinant viruses for cytotoxicity in fibroblast and lymphocyte cultures. Both the fibrotropic and lymphotropic determinants mapped to the same 237-nucleotide sequence within the coding region of the virus structural gene. A second sequence, near the viral promoter at map unit 38, was also shown to affect viral growth in fibroblast host cells profoundly.     

DNA sequence comparison between two tissue-specific variants of the autonomous parvovirus, minute virus of mice.

We have determined the complete nucleotide sequence of the DNA of the immunosuppressive variant of the parvovirus minute virus of mice (MVMi) and compared it to the published sequence (12) of the fibroblast-specific strain (MVMp). We have found 175 differences between the two viruses, most of which affect single nucleotides. Despite these differences, the genomic organization of MVMp and MVMi is identical. There are 29 amino-acid changes between the putative viral gene products of MVMi and MVMp, 16 of which are conservative. We discuss the possibility that the differential tissue-specificity of the two variants is linked to differences within the non-transcribed region near the 5' end of the viral genomes.     

Replication initiator protein NS1 of the parvovirus minute virus of mice binds to modular divergent sites distributed throughout duplex viral DNA

To initiate DNA synthesis, the NS1 protein of minute virus of mice (MVM) first binds to a simple cognate recognition sequence in the viral origins, comprising two to three tandem copies of the tetranucleotide TGGT. However, this motif is also widely dispersed throughout the viral genome. Using an immunoselection procedure, we show that NS1 specifically binds to many internal sites, so that all viral fragments of more than ~170 nucleotides effectively compete for NS1, often binding with higher affinity to these internal sites than to sites in the origins. We explore the diversity of the internal sites using competitive binding and DNase I protection assays and show that they vary between two extreme forms. Simple sites with three somewhat degenerate, tandem TGGT reiterations bind effectively but are minimally responsive to ATP, while complex sites, containing multiple variably spaced TGGT elements arranged as opposing clusters, bind NS1 with an affinity that can be enhanced ~10-fold by ATP. Using immuno-selection procedures with randomized sequences embedded within specific regions of the genome, we explore possible binding configurations in these two types of site. We conclude that binding is modular, combinatorial, and highly flexible. NS1 recognizes two to six variably spaced, more-or-less degenerate forms of the 5′-TGGT-3′ motif, so that it binds efficiently to a wide variety of sequences. Thus, despite complex coding constraints, binding sites are configured at frequent intervals throughout duplex forms of viral DNA, suggesting that NS1 may serve as a form of chromatin to protect and tailor the environment of replicating genomes.     

Structural determinants of tissue tropism and in vivo pathogenicity for the parvovirus minute virus of mice

Two strains of the parvovirus minute virus of mice (MVM), the immunosuppressive (MVMi) and the prototype (MVMp) strains, display disparate in vitro tropism and in vivo pathogenicity. We report the crystal structures of MVMp virus-like particles (MVMp(b)) and native wild-type (wt) empty capsids (MVMp(e)), determined and refined to 3.25 and 3.75 A resolution, respectively, and their comparison to the structure of MVMi, also refined to 3.5 A resolution in this study. A comparison of the MVMp(b) and MVMp(e) capsids showed their structures to be the same, providing structural verification that some heterologously expressed parvovirus capsids are indistinguishable from wt capsids produced in host cells. The structures of MVMi and MVMp capsids were almost identical, but local surface conformational differences clustered from symmetry-related capsid proteins at three specific domains: (i) the icosahedral fivefold axis, (ii) the "shoulder" of the protrusion at the icosahedral threefold axis, and (iii) the area surrounding the depression at the icosahedral twofold axis. The latter two domains contain important determinants of MVM in vitro tropism (residues 317 and 321) and forward mutation residues (residues 399, 460, 553, and 558) conferring fibrotropism on MVMi. Furthermore, these structural differences between the MVM strains colocalize with tropism and pathogenicity determinants mapped for other autonomous parvovirus capsids, highlighting the importance of common parvovirus capsid regions in the control of virus-host interactions.     

Evidence for a ligation step in the DNA replication of the autonomous parvovirus minute virus of mice

Newly replicated DNA of the autonomous parvovirus minute virus of mice was pulse-labeled with 32PO4 during the time of maximal viral DNA replication in highly synchronized A9 cells. The subsequent processing of viral DNA-protein complexes was monitored during a chase period with no label. Several distinct classes of duplex replicative-form and progeny single-stranded DNA molecules were characterized and found to accumulate at different times during infection. Analysis of the terminal structures associated with these various forms provided new insights into the mechanism by which viral DNA replicates and, in particular, suggested that interstrand ligation occurs during this process.     

Specific binding sites for a parvovirus, minute virus of mice, on cultured mouse cells.

The early interactions between parvoviruses and host cells have not been extensively described previously. In this study we have characterized some aspects of viral binding to the cell surface and demonstrated the existence of specific cellular receptor sites for minute virus of mice (MVM) on two murine cell lines that are permissive for viral growth. The interaction had a pH optimum of 7.0 to 7.2, and both the rate and extent of the reactions were slightly affected by temperature. Mouse A-9 cells (L-cell derivative) had approximately 5 X 10(5) specific MVM binding sites per cell, and Friend erythroleukemia cells had 1.5 X 10(5) MVM sites per cell. In contrast, the nonpermissive mouse lymphoid cell line L1210 lacked specific viral receptors. Also, cloned lines of A-9 cells resistant to viral infection have been isolated. One of these lines lacked the "specific" virus attachment sites but exhibited low levels of nonsaturable virus binding. Based on these examples, infectivity is correlated with the presence of specific viral receptors on the cell surface.     

Virulent variants emerging in mice infected with the apathogenic prototype strain of the parvovirus minute virus of mice exhibit a capsid with low avidity for a primary receptor

The mechanisms involved in the emergence of virulent mammalian viruses were investigated in the adult immunodeficient SCID mouse infected by the attenuated prototype strain of the parvovirus Minute Virus of Mice (MVMp). Cloned MVMp intravenously inoculated in mice consistently evolved during weeks of subclinical infection to variants showing altered plaque phenotypes. All the isolated large-plaque variants spread systemically from the oronasal cavity and replicated in major organs (brain, kidney, liver), in sharp contrast to the absolute inability of the MVMp and small-plaque variants to productively invade SCID organs by this natural route of infection. The virulent variants retained the MVMp capacity to infect mouse fibroblasts, consistent with the lack of genetic changes across the 220-to-335 amino acid sequence of VP2, a capsid domain containing main determinants of MVM tropism. However, the capsid of the virulent variants shared a lower affinity than the wild type for a primary receptor used in the cytotoxic infection. The capsid gene of a virulent variant engineered in the MVMp background endowed the recombinant virus with a large-plaque phenotype, lower affinity for the receptor, and productive invasiveness by the oronasal route in SCID mice, eventually leading to 100% mortality. In the analysis of virulence in mice, both MVMp and the recombinant virus similarly gained the bloodstream 1 to 2 days postoronasal inoculation and remained infectious when adsorbed to blood cells in vitro. However, the wild-type MVMp was cleared from circulation a few days afterwards, in contrast to the viremia of the recombinant virus, which was sustained for life. Significantly, attachment to an abundant receptor of primary mouse kidney epithelial cells by both viruses could be quantitatively competed by wild-type MVMp capsids, indicating that virulence is not due to an extended receptor usage in target tissues. We conclude that the selection of capsid-receptor interactions of low affinity, which favors systemic infection, is a major evolutionary process in the adaptation of parvoviruses to new hosts and in the cause of disease.