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Journal of Virology, October 2004, p. 11411-11415, Vol. 78, No. 20
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.20.11411-11415.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Visualization by Live-Cell Microscopy of Disruption of ND10 during Herpes Simplex Virus Type 1 Infection
Roger D. Everett* and Alexandros Zafiropoulos
Medical Research Council Virology Unit, University of Glasgow, Glasgow, United Kingdom
Received 3 April 2004/
Accepted 1 June 2004

ABSTRACT
ND10 structures are disrupted during herpes simplex virus type
1 (HSV-1) infection by viral regulatory protein ICP0. The significance
of this effect remains controversial, partly because of a report
that high-level expression of the major ND10 promyelocytic leukemia
(PML) protein precludes ND10 disruption yet does not inhibit
HSV-1 infection. Here we demonstrate dramatic reorganization
of ND10 during HSV-1 infection by live-cell microscopy, even
in the presence of overexpressed PML.

TEXT
ND10 (also known as promyelocytic leukemia [PML] nuclear bodies)
are discrete nuclear foci that contain several cellular proteins
involved in diverse processes such as transcription, chromatin
structure, DNA repair, and apoptosis (
1,
7,
23,
24,
30). ND10
are dynamic structures that disassemble during mitosis and apoptosis
and are structurally affected by many forms of stress, such
as heat shock, heavy metal treatment, and a variety of drugs
that alter cellular metabolism. ND10 are also intimately associated
with the replication of many nuclear replicating DNA viruses
in that parental viral genomes frequently associate with ND10,
and it is at these locations that viral immediate-early (IE)
gene transcription occurs and from which viral DNA replication
compartments develop (
5,
18,
20,
22,
29; reviewed in references
8 and
21). In the case of herpes simplex virus type 1 (HSV-1),
recent evidence suggests that the association of viral genomes
with ND10 may be functionally important, because genomes that
are so associated in the initial stages of infection have an
increased probability of developing into a replication compartment
(
14,
15,
28). Further evidence for a role of ND10 in HSV-1 replication
comes from a strong implication that ND10-like structures assemble
in association with and in response to incoming HSV-1 genomes
in the absence of IE regulatory protein ICP0 (
14). ICP0 is a
ubiquitin E3 ligase that brings about the destruction of ND10
in wild-type HSV-1 infections through inducing the degradation
of the principal core component of ND10, the PML protein. The
ability of ICP0 to disrupt ND10 by this mechanism correlates
very well with its role in stimulating HSV-1 lytic infection
and reactivation from quiescence or latency (
2,
3,
11,
16; for
reviews see references
9,
17,
26, and
27). All the above evidence
suggests but does not prove that ND10 structures have important
roles in HSV-1 infection.
In contrast, it has been reported that high-level expression of PML by using a baculovirus engineered to express proteins in mammalian cells leads to the formation of large ND10 complexes that are not disrupted during HSV-1 infection (19). The presence of very high levels of PML in this situation did not impede HSV-1 infection (19), a conclusion that is consistent with that of a previous study (6) and with unpublished data from this laboratory. Therefore, there is consistent evidence that high levels of PML do not inhibit HSV-1 infection, at least in the situations so far examined. However, because large ND10-like structures remained during HSV-1 infection of cells expressing very high levels of PML, it was concluded that the disruption of ND10 has no functional role (19). If true, this is a highly important conclusion, because it places in doubt the significance of a substantial body of published work and interpretation that is relevant not only to HSV-1 but also to a spectrum of DNA viruses. On the other hand, treatments that inhibit the disruption of ND10 by ICP0 inhibit the formation of replication compartments, progression to efficient early gene expression, and production of viral progeny in low-multiplicity HSV-1 infections (4, 5, 13). In view of this apparently conflicting evidence, this study was initiated to test the hypothesis that high-level expression of PML resulted in ND10 structures that were resistant to disruption during HSV-1 infection. Instead of relying on extrapolations from fixed-cell images, the fate of ND10 and PML protein distribution was followed by time-lapse microscopy of live infected cells.
Baculovirus Ac.CMV.EYFP-PML contains the PML (isoform IV) cDNA with an N-terminal enhanced yellow fluorescent protein (EYFP) tag downstream of the human cytomegalovirus (HCMV) IE promoter/enhancer (15). This isoform of PML was chosen because it gives a pattern of SUMO-modified forms that bear a strong resemblance to those of the endogenous protein and because, like endogenous PML (11, 25), its sensitivity to the effects of ICP0 have been well characterized (2, 25). Analogous baculoviruses expressing Sp100 (isoform A) and hDaxx were constructed (Ac.CMV.EYFP-Sp100 and Ac.CMV.EYFP-Daxx). Infection of Vero cells with these viruses indicated that all three proteins were expressed at very high levels (estimated to be of the order of 100-fold in excess of the endogenous proteins), and in the case of both PML and Sp100, modified forms consistent with the normal conjugation of these proteins to SUMO-1 were evident (Fig. 1). Vero cells in a coverslip glass chamber unit were coinfected with Ac.CMV.ECFP-PML (28) and Ac.CMV.EYFP-Sp100, and the following day the unit was placed in a live-cell microscopy suite equipped with full environmental control and motorized accessories for detection of EYFP and enhanced cyan fluorescent protein (ECFP) in the same sample (15). Control experiments showed that in uninfected cells the fluorescent PML and Sp100 proteins extensively colocalized in ND10 structures that, in interphase cells, did not show extensive changes in appearance or composition over the time periods used for the following experiments (data not shown).
Additional wells of Vero cells infected with Ac.CMV.ECFP-PML
and Ac.CMV.EYFP-Sp100 were prepared, and the next day the cells
were infected with HSV-1 strain 17 (multiplicity of infection
[MOI], 10). Soon after virus adsorption the sample was placed
in the microscope system, and then fields of view were chosen
and images were captured at 10-min intervals for periods of
3 to 4 h. Selected images from a typical time sequence are shown
in Fig.
2 and
3, and the complete movie is available in the
supplementary data (Movie S1). Consistent with previous data
(
19), high-level expression of PML caused the assembly of large
ND10-like foci, and most cells infected with both baculoviruses
contained both PML and Sp100. As HSV-1 infection progressed,
Sp100 was lost from the ND10-like foci and the proportion of
Sp100 that was diffusely spread through the nucleus increased.
While some punctate PML structures remained throughout, they
clearly underwent a dramatic rearrangement during the first
few hours of infection. Movie S1 shows that the foci became
less distinct and even appeared to go out of focus before becoming
more distinct again towards the end of the sequence. While movement
of some of the foci out of the focal plane cannot be discounted,
this type of behavior occurred in all infected cells examined
by time-lapse microscopy, and it did not occur in uninfected
cells. Visual inspection of the samples confirmed the reduction
in numbers of PML foci during infection, illustrating that the
results were not due to loss of camera focus. Rather, the PML
foci became less distinct, disaggregated, and highly mobile,
and then the more stable distinct structures that were formed
towards the end of the sequence appeared to arise from the merging
of PML material from ND10 remnants. Similar results were obtained
with Vero cells coinfected with baculoviruses Ac.CMV.ECFP-PML
and Ac.CMV.EYFP-Daxx (Fig.
2 and
3). We conclude that although
large foci of PML can be retained after overexpression of PML
in HSV-1-infected cells, the PML structures undergo substantial
changes in both composition (loss of Sp100 and hDaxx) and morphology
(disaggregation and then merging of remnant material). Although
it was not possible to directly identify HSV-1-infected cells
simultaneously in these experiments, controls using fixed cells
and antibody staining confirmed that all the Vero cells became
HSV-1 infected at this MOI (data not shown). The changes that
we have observed here would not have been detectable in fixed-cell
experiments, because fixation does not allow the fate of individual
PML foci to be followed over time.
The fate of the overexpressed ND10 proteins during infection
of Vero cells was investigated by Western blotting. Vero cells
were infected with baculoviruses expressing either EYFP alone
or the EYFP-PML, EYFP-Sp100, or EYFP-hDaxx fusion proteins.
The MOIs used were the same as those in the live-cell studies.
The following day replicate samples were infected with HSV-1
strain 17 (MOI, 10) and were harvested 2, 4, and 6 h later.
After sodium dodecyl sulfate-polyacrylamide gel electrophoresis
and Western blotting, the filters were probed with an anti-EYFP
antibody and an antibody to detect the viral DNA replication
protein UL42, a marker for the progression of the infection.
Figure
4 shows that infection with HSV-1 caused a slight reduction
in the major PML band (presumed to be unmodified by SUMO-1),
but there was a marked reduction in the slower migrating SUMO-modified
forms. This observation is consistent with the study of Lopez
et al. (
19) and presumably reflects the disruption of the ND10
structures that we have detected in this study. In the case
of Sp100, there were relatively minor changes in the comparatively
less abundant lower mobility forms and, rather than any decrease
in the level of the major Sp100 band, there was a modest increase
during HSV-1 infection that is likely due to the HSV-1 transactivators
enhancing expression from the HCMV promoter in the baculovirus
genome nonspecifically. This behavior contrasts with the loss
of endogenous Sp100 seen during HSV-1 infection, although even
in this case the loss is preferentially of the presumed SUMO-modified
forms (
25). Other than a similar transactivation effect, we
did not observe any major changes to the levels of baculovirus-expressed
hDaxx during HSV-1 infection (Fig.
4), consistent with the behavior
of endogenous hDaxx (data not shown). High-level expression
of EYFP-PML or the other fusion proteins did not compromise
the efficiency of UL42 expression by HSV-1 in high-multiplicity
infections of Vero cells (Fig.
4), in agreement with the conclusions
of Lopez et al. (
19).
To test whether this was a cell-type-specific phenomenon and
to provide a direct control for virus infection, human fibroblast
HFFF-2 cells were infected with Ac.CMV.EYFP-PML alone and the
fate of the PML foci was followed after infection with HSV-1
virus vECFP-ICP4, which expresses viral IE protein ICP4 linked
to ECFP (
14,
15). ICP4 is a transcriptional regulator that associates
with parental viral genomes and accumulates in viral replication
centers (
14 and references therein). Again, although PML foci
remained throughout the infection period, there was a dramatic
reorganization of the PML protein followed by its accumulation
into a reduced number of foci (Fig.
2 and
3 and Movie S2). Although
the remaining PML foci appear to arise from the merging of PML
material from the original structures, the interpretation of
these image sequences requires some caution because of the uncertainties
introduced by the time intervals between the images and possible
movement of foci from the optimal focal plane. With these limitations
in mind, it nonetheless appears that after infection many of
the PML foci become surrounded by increased diffuse fluorescence
in the vicinity of the original structure, suggesting a marked
efflux of PML protein. At later times this diffuse fluorescence
is less marked and the PML foci become distinct and well-defined
again, superficially similar to their appearance before infection.
The reorganization of the PML foci began at an early stage of
infection, before extensive synthesis of ICP4 was detectable
by the image capture conditions used. Note, however, that detection
of ECFP-ICP4 is not very sensitive in the live-cell microscopy
system; staining of fixed cells by using antibodies readily
detects ICP4 as early as 1 h after infection of Vero cells at
an MOI of 10 (data not shown). The examples shown in Fig.
2 and
3 focus on cells that contain large but not exceptionally
extensive PML aggregates. However, during the course of these
experiments it was observed that even the largest PML aggregates
were subject to disruption during HSV-1 infection in both Vero
and HFFF-2 cells (data not shown).
These experiments clearly show that high-level expression of PML produces large aggregates of PML that nonetheless are subject to modification in terms of both content and morphology during HSV-1 infection. While overexpression of PML itself does not impede HSV-1 infection (6, 19), the conclusion that ND10 disruption is not important for productive HSV-1 infection (19) is not proven. It is possible that disruption of PML structures through the ubiquitin E3 ligase activity of ICP0 releases or changes the activity of activators and/or repressors that impinge on the efficiency of HSV-1 infection. Thus, the targeting of PML and ND10 by ICP0 may not be to relieve an inhibition mediated by PML itself, but more of a means to affect other factors that are ND10 associated. Accordingly, high-level expression of PML may not impede HSV-1 infection, because it does not abrogate ND10 disruption and the resultant downstream effects on these other factors. On the other hand, the observations reported here do not prove that ND10 disruption is an important or essential process during HSV-1 infection.
It will remain extremely difficult to design experiments that assess directly the role of ND10 disruption in the progression of HSV-1 infection. We note that in these experiments and in several previous studies (4, 5), and even in the absence of ICP0 (14), the commitment of cells into the HSV-1 lytic cycle and the development of viral replication compartments is accompanied by significant changes in the morphology of ND10. Whether the processes of viral genome association with ND10 and subsequent ND10 disruption underlie positive or negative influences on HSV-1 replication (or elements of both) or are merely effects without functional significance remains to be determined; these issues have been discussed in detail elsewhere (10, 14). Whatever the solution to these complicated questions, it is clear that at the descriptive level, modification or disruption of ND10 is intimately linked to the commitment of cells to lytic HSV-1 infection and the development of replication compartments.

ACKNOWLEDGMENTS
This work was supported by the Medical Research Council and
by the award of a short-term fellowship to A.Z. by the European
Molecular Biology Organization.
We are grateful for the support of Demetrios Spandidos (Crete Medical School) in enabling A.Z. to take up the EMBO fellowship in Glasgow. We thank Gerd Maul (Wistar Institute) for providing an hDaxx cDNA clone and Wei-Li Hsu and Anne Orr for unpublished work investigating whether high-level expression of ND10 proteins impedes HSV-1 infection.

FOOTNOTES
* Corresponding author. Mailing address: MRC Virology Unit, Church St., University of Glasgow, Glasgow G11 5JR, United Kingdom. Phone: 44 141 330 3923. Fax: 44 141 337 2236. E-mail:
r.everett{at}vir.gla.ac.uk.

Supplemental material for this article may be found at http://jvi.asm.org/. 
Present address: Laboratory of Histology, Faculty of Medicine, University of Crete, Heraklion 71110-GR, Greece. 

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Journal of Virology, October 2004, p. 11411-11415, Vol. 78, No. 20
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.20.11411-11415.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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