Polyomavirus genome and polyomavirus enhancer-driven gene expression during myogenesis

Mouse polyomavirus late region mutants expressing a defective VP2 capsid protein exhibit an enhanced viral DNA replication

We have constructed mouse polyomavirus mutants with deletions or insertions in the late region. These viral constructs exhibit two different phenotypes, characterized by a differential replication ability. The first type shows a twofold higher level of DNA synthesis with respect to wild type, while mutants of the second type are virtually unable to replicate. In this paper we investigate the replicative behavior of the first class of mutants, which expresses defective viral coat proteins. Our data raise the possibility that, of the three late gene viral products, VP2 is involved in the formation of an encapsidation intermediate that, in an indirect fashion, affects the rate of viral DNA synthesis.

Viral genomes maintained extrachromosomally in hamster polyomavirus-induced lymphomas display a cell-specific replication in vitro

Hamster polyomavirus causes lymphomas when injected into newborn Syrian hamsters. Large amounts of extrachromosomal viral genomes are accumulated in the lymphoma cells. These genomes are characterized by deletions affecting the late coding region as well as a specific part of the noncoding regulatory region. By contrast with wild-type genomes, lymphoma-associated genomes replicate in a lymphoblastoid cell line but not in a fibroblastic cell line. The deletion acts in a cis-dominant manner and is the primary determinant of this host-range effect on replication. The boundaries of the regulatory region necessary for viral DNA replication in the two cell contexts have been defined. The regulatory region can be functionally divided in two domains: one domain (distal from the origin of replication) is necessary for viral genome replication in fibroblasts, whereas the other domain (proximal to the origin of replication) is functional only in the lymphoblastoid cell context and contains the sequence specifically conserved in the lymphoma-associated genomes. This sequence harbors a motif recognized by a lymphoblastoid cell-specific trans-acting factor.

Aphidicolin-resistant polyomavirus and subgenomic cellular DNA synthesis occur early in the differentiation of cultured myoblasts to myotubes

Small DNA viruses have been historically used as probes of cellular control mechanisms of DNA replication, gene expression, and differentiation. Polyomavirus (Py) DNA replication is known to be linked to differentiation of may cells, including myoblasts. In this report, we use this linkage in myoblasts to simultaneously examine (i) cellular differentiation control of Py DNA replication and (ii) an unusual type of cellular and Py DNA synthesis during differentiation. Early proposals that DNA synthesis was involved in the induced differentiation of myoblasts to myotubes were apparently disproved by reliance on inhibitors of DNA synthesis (cytosine arabinoside and aphidicolin), which indicated that mitosis and DNA replication are not necessary for differentiation. Theoretical problems with the accessibility of inactive chromatin to trans-acting factors led us to reexamine possible involvement of DNA replication in myoblast differentiation. We show here that Py undergoes novel aphidicolin-resistant net DNA synthesis under specific conditions early in induced differentiation of myoblasts (following delayed aphidicolin addition). Under similar conditions, we also examined uninfected myoblast DNA synthesis, and we show that soon after differentiation induction, a period of aphidicolin-resistant cellular DNA synthesis can also be observed. This drug-resistant DNA synthesis appears to be subgenomic, not contributing to mitosis, and more representative of polyadenylated than of nonpolyadenylated RNA. These results renew the possibility that DNA synthesis plays a role in myoblast differentiation and suggest that the linkage of Py DNA synthesis to differentiation may involve a qualitative cellular alteration in Py DNA replication.

Adult mouse kidneys become permissive to acute polyomavirus infection and reactivate persistent infections in response to cellular damage and regeneration

Kidneys of newborn (but not adult) mice are normally high permissive for polyomavirus (Py) infection and readily establish persistent infections. We have proposed that ongoing cellular differentiation, which occurs in newborn mice, may be necessary for a high level of in vivo Py replication (R. Rochford, J. P. Moreno, M. L. Peake, and L. P. Villarreal, J. Virol. 66:3287-3297, 1992). This cellular differentiation requirement may also be necessary for the reactivation of a persistent Py kidney infection and could provide an alternative to the accepted view that reactivation results from immunosuppression. To examine this proposal, the ability of adult BALB/c mouse kidneys to support primary acute Py infection or to reactivate previously established persistent Py infections after kidney-specific damage was investigated. Kidney damage was induced by both chemical (glycerol, cisplatin, or methotrexate) and mechanical (through renal artery clamping to produce unilateral renal ischemia) treatments. We also examined the effects of epidermal growth factor (EGF), which enhances the rate of kidney regeneration, on Py replication. Using histopathologic techniques, in situ hybridization for Py DNA, and immunofluorescence for Py VP1 production, we established that both chemical damage and damage through renal artery clamping of adult kidneys promoted high levels of primary Py replication in these normally nonpermissive cells. This damage also promoted the efficient reactivation of Py replication from persistently infected kidneys, in the absence of immunosuppression. EGF treatment significantly increased acute Py replication and also reactivation in damaged kidneys. These results support the view that ongoing cellular division and differentiation may be needed both for high levels of acute Py replication and for reactivation of persistent infections in vivo.

Mutations in the VP1 coding region of polyomavirus determine differentiating stage specificity

Polyomavirus mutants capable of replicating in undifferentiated murine C2 myoblasts were selected and characterized. These mutants grow normally in 3T6 mouse fibroblast cells, and they do not complement the wild-type virus in coinfection experiments of C2 myoblasts. Of 12 isolates, 10 possess duplications of the regulatory region including the enhancer A domain. On the bases of the regulatory region structure and the presence and length of the enhancer duplication, the mutant viruses could be grouped into three classes. One mutant class (e.g., PyMB3) possesses an enhancer duplication of 91 bp identical to that of a previously characterized polyomavirus mutant, PyNB11/1. We have demonstrated that this enhancer duplication gives rise at its junction to a novel recognition motif for the transcriptional factor NF-1 (M. Caruso, C. Iacobini, C. Passananti, A. Felsani, and P. Amati, EMBO J. 9:947-955, 1990). The regulatory region PyMB3 virus recombined in a wild-type genome context maintains the mutant phenotype. The other two types of mutants, one with a 30-bp enhancer duplication (e.g., PyMB40) and one with a wild-type enhancer structure (e.g., PyMB27), possess two similar but distinct 6-bp deletions in the same region of the VP1 coding gene. In both cases, the ability to replicate in undifferentiated C2 myoblasts is strictly correlated to the mutation in the VP1 coding region.

Relationship of eukaryotic DNA replication to committed gene expression: general theory for gene control

The historic arguments for the participation of eukaryotic DNA replication in the control of gene expression are reconsidered along with more recent evidence. An earlier view in which gene commitment was achieved with stable chromatin structures which required DNA replication to reset expression potential (D. D. Brown, Cell 37:359-365, 1984) is further considered. The participation of nonspecific stable repressor of gene activity (histones and other chromatin proteins), as previously proposed, is reexamined. The possible function of positive trans-acting factors is now further developed by considering evidence from DNA virus models. It is proposed that these positive factors act to control the initiation of replicon-specific DNA synthesis in the S phase (early or late replication timing). Stable chromatin assembles during replication into potentially active (early S) or inactive (late S) states with prevailing trans-acting factors (early) or repressing factors (late) and may asymmetrically commit daughter templates. This suggests logical schemes for programming differentiation based on replicons and trans-acting initiators. This proposal requires that DNA replication precede major changes in gene commitment. Prior evidence against a role for DNA replication during terminal differentiation is reexamined along with other results from terminal differentiation of lower eukaryotes. This leads to a proposal that DNA replication may yet underlie terminal gene commitment, but that for it to do so there must exist two distinct modes of replication control. In one mode (mitotic replication) replicon initiation is tightly linked to the cell cycle, whereas the other mode (terminal replication) initiation is not cell cycle restricted, is replicon specific, and can lead to a terminally differentiated state. Aberrant control of mitotic and terminal modes of DNA replication may underlie the transformed state. Implications of a replicon basis for chromatin structure-function and the evolution of metazoan organisms are considered.

Reduced Polyomavirus DNA replication as a consequence of a late-region deletion that results in early mRNA instability

A Polyomavirus mutant with a 504 base pair deletion downstream of the polyadenylation signal for the early genes was constructed in vitro. This mutant showed a reduced synthesis of viral DNA. In cotransfection experiments, this defect was complemented by the presence of a wild-type genome. To define the sequences involved in the determination of this phenotype, a set of viral mutants was constructed. The properties of these mutants suggested that the deletion of a short DNA segment located 35 base pairs downstream of the early polyadenylation site affected the stability of early mRNA. The boundaries of the deletion were within the late coding sequences. However, the truncated form of the major capsid protein VP1 expressed by the mutant, did not influence the formation of early mRNA and the synthesis of viral DNA.