Previous Article | Next Article 
Journal of Virology, February 2006, p. 2063-2068, Vol. 80, No. 4
0022-538X/06/$08.00+0 doi:10.1128/JVI.80.4.2063-2068.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Deacetylation of the Herpes Simplex Virus Type 1 Latency-Associated Transcript (LAT) Enhancer and a Decrease in LAT Abundance Precede an Increase in ICP0 Transcriptional Permissiveness at Early Times Postexplant
Antonio L. Amelio,
Nicole V. Giordani,
Nicole J. Kubat,
Jerome E. O'Neil, and
David C. Bloom*
Department of Molecular Genetics and Microbiology, University of Florida College of Medicine, Gainesville, Florida 32610-0266
Received 6 September 2005/
Accepted 22 November 2005

ABSTRACT
Only the latency-associated transcript (LAT) of the herpes simplex
virus type 1 (HSV-1) genome is transcribed during latency, while
the lytic genes are suppressed, possibly by LAT antisense mechanisms
and/or chromatin modifications. In the present study, latently
infected dorsal root ganglia were explanted to assess both relative
levels of LAT and histone H3 (K9, K14) acetylation of the LAT
locus and ICP0 promoter at early times postexplant. We observed
that a decrease in both LAT enhancer histone H3 (K9, K14) acetylation
and LAT RNA abundance occurs prior to an increase in acetylation,
or transcriptional permissiveness, at the ICP0 promoter.

TEXT
Herpes simplex virus type 1 (HSV-1) is characterized by its
ability to establish latency as an episome in neurons (
12).
During this time, transcriptional activity is virtually nonexistent,
with the exception of the latency-associated transcript (LAT),
an 8.3- to 8.5-kb noncoding RNA that can be spliced to yield
a 2.0-kb stable intron (
5,
13,
16). One proposed function of
the LAT is the suppression of nearby lytic phase transcripts
ICP0,

34.5, and ICP4 through antisense mechanisms, thereby promoting
the establishment and maintenance of latency (
3). Studies using
LAT promoter and/or 5' exon mutants demonstrate impaired establishment
of latency and leaky expression of lytic phase transcripts (
3).
In addition, a recent study has demonstrated that the lytic
gene regions of LAT mutants are associated with less of the
repressive histone H3 K9 dimethyl, suggesting that the LAT plays
a direct role in promoting a transcriptionally nonpermissive
environment for lytic genes during latency (
18). If the LAT
is indeed responsible for suppressing lytic phase transcripts
during latency, one might expect reactivation to directly regulate
LAT levels.
In addition to putative LAT-mediated suppression of lytic transcripts during latency, mounting evidence suggests that latent gene expression is also regulated at the chromatin level. The latent viral genome is known to associate with nucleosomes (4). Investigation of chromatin modification, in particular, the acetylation of histone H3 lysine residues 9 and 14 (K9 and K14, respectively), demonstrates that during latency, the lytic regions of the virus exist in a hypoacetylated, or transcriptionally nonpermissive, state, while the LAT promoter and 5' exon/enhancer remain hyperacetylated, or transcriptionally permissive (9) (Fig. 1). However, LAT transcription is not a prerequisite, nor is it necessary to maintain the hyperacetylated, or structurally relaxed, chromatin state (8), suggesting that the enhancer within the LAT region is an important cis-acting DNA element.
Reactivation of the latent viral genome has been linked to a
reactivation critical region (
rcr) that encompasses the LAT
core promoter through the LAT 5' exon/enhancer, since recombinants
lacking this region display greatly reduced reactivation phenotypes
(
2,
6,
7,
10). However, this still does not address whether
the regulatory elements in the
rcr act at the RNA or DNA/chromatin
level. An initial study using Northern blot analysis had detected
a decrease in LAT at 24 and 36 h postexplant of latently infected
murine trigeminal ganglia (
14), suggesting that LAT expression
may be incompatible with reactivation. Therefore, we sought
to determine whether changes in LAT transcription and/or histone
acetylation occur at early times during reactivation.
A model to study early molecular events of reactivation involves explant of latently infected murine dorsal root ganglia (DRG) into supplemented medium, a process that results in reactivation of latent virus (15). In the present study, explanted DRG were placed in medium and incubated for specific time intervals, followed by RNA isolation or chromatin cross-linking.
To determine the effect of explant on LAT levels, RNA was isolated from female murine DRG and reverse transcribed using random decamers, and the resulting cDNA was analyzed by real-time (TaqMan) PCR. Relative quantities of the LAT were normalized to either adenine phosphoribosyl transferase (APRT), a cytoplasmic cellular transcript, or to Xist, a nuclear noncoding RNA. As shown in Fig. 2, LAT abundance may transiently increase initially (1.5- to 2-fold), before decreasing between 2 and 3 h postexplant (hpe). Regardless of the cellular control used for normalization, the overall pattern of expression appears the same, suggesting that the decreases observed for the LAT following explant were not due to a general decrease in either total cellular or nuclear RNAs.
The observed effect of explant on LAT RNA levels suggests that
early events in explant-induced reactivation may alter transcription
of the LAT. Previously, we reported that the LAT region is maintained
in a hyperacetylated state during latency, independently of
LAT transcription; therefore, we sought to determine whether
there was a change in histone H3 (K9, K14) acetylation following
explant. Chromatin immunoprecipitation (ChIP) analysis of the
LAT promoter following explant demonstrated a dramatic reduction
in acetylation as early as 1 hpe (Fig.
3A). In order to be certain
that this effect was not due to global changes in histone acetylation
caused by explant-induced stress, subsequent experiments (Fig.
3B and C) were normalized to APRT, a constitutively expressed
cellular gene. In the third experiment (Fig.
3C), the decrease
in acetylation at the 1-hpe time point was not as dramatic,
and in this experiment the acetylation actually increased between
1 and 2 hpe. Overall, the LAT promoter displayed a variable
decrease in histone H3 acetylation occurring within the first
hour of explant.
Because it was previously reported that the LAT enhancer is
capable of increasing LAT transcription (
1) and because acetylation
of the enhancer was shown to be independent of abundant LAT
transcription (
8), it seemed plausible that the observed decrease
in LAT abundance might correlate with changes in the acetylation
of histones associated with the LAT enhancer following explant-induced
reactivation. Real-time PCR analyses of the LAT 5' exon for
the same three ChIP experiments that were analyzed for the LAT
promoter displayed marked decreases of at least fivefold in
acetylation occurring as early as 0.5 hpe (Table
1; Fig.
3A to C),
with the decrease by 1 hpe being statistically significant
among the three independent ChIP experiments (
P > 0.02).
It should be noted that in experiment 3, following a dramatic
decrease in acetylation of the 5' exon at 0.5 hpe, there is
an increase in acetylation. This increase parallels the increase
in acetylation observed at the LAT promoter in this same experiment
and possibly reflects a more rapid return to the "latent" acetylation
state in this set of ganglia. Overall, the dramatic and rapid
decreases in the acetylation of histone H3 associated with the
5' exon and LAT promoter suggest that a rapid change in transcriptional
permissiveness precedes the decrease in LAT RNA abundance.
To determine whether the observed changes in the LAT abundance
are linked to a change in the transcriptional permissiveness
of the ICP0 promoter, we also analyzed its acetylation status
using the same ChIP experiments. As shown in Fig.
3A to C (far
right column), there is a net increase in ICP0 promoter acetylation
occurring as early as 2 hpe and increasing by as much as threefold
by 3 hpe. Despite this increase in acetylated histone H3 associated
with the ICP0 promoter, no significant increase in ICP0 transcription
could be detected by 4 hpe (data not shown). Taken together,
these data show a sequential process where changes in chromatin
structure of the LAT enhancer and decreased transcription of
the LAT allow for an increase in acetylation at the ICP0 promoter,
perhaps facilitating productive reactivation in at least some
neurons.
In order to extend these analyses and determine whether an increase in ICP0 transcription could be detected following longer incubations of the explants, we performed three independent ChIP and reverse transcription-PCR (RT-PCR) experiments at 8 and 12 hpe. We chose 12 hpe as our latest time point since it has been recently shown that infectious virus is first detected in the majority of explants at 14 hpe (11). ChIP analyses of the 8- and 12-hpe time points revealed that the LAT and ICP0 promoters and the LAT 5' exon show comparable levels of acetylation at 8 and 12 h (Fig. 4A). In comparison to the levels of acetylation observed at 3 and 4 h, this represents a slight decrease in transcriptional permissiveness of the ICP0 promoter but is still higher than the baseline values for time zero (latent expression). RT-PCR analyses of these later time points clearly demonstrate that the levels of LAT RNA remain low through the 12-h time point (Fig. 4B). These results are consistent with, and extend, our observations at the earlier time points (Fig. 2). Nonetheless, we still failed to detect a significant increase in ICP0 transcription by 12 hpe. It should be noted that the earliest detection of a net increase in ICP0 transcripts in explanted ganglia was 96 h by Northern blotting (14) or 24 h by RT-PCR (17). Since it has been shown that the first round of reactivating virus produced by explant occurs by 14 h (11), it is likely that the inability to detect an increase in ICP0 RNA is because ICP0 transcription occurs in only the small subset of cells that ultimately go down the path to productive reactivation. It is well documented that reactivation occurs only in a small percentage of the total population of latently infected neurons, so detection of this small increase in ICP0 transcripts above the background of the entire latent population would be difficult, if not impossible. It is also likely that the ICP0 transcription detected at 24 h and later postexplant reflects secondary rounds of replication within the explanted ganglia, possibly in nonneuronal support cells (11).
This study sought to determine the relationship between regulation
of the LAT and the LAT region acetylation status at early reactivation
times. The observations described here support the hypothesis
that the LAT may act to suppress immediate-early genes and that
the chromatin status of the LAT enhancer is linked to early
reactivation events. Previous studies using the LAT promoter
and/or 5' exon mutants showed impaired establishment of latency
and leaky expression of lytic phase transcripts during latency.
Our finding that a significant and dramatic decrease in the
LAT occurs at early times during explant-induced reactivation
supports the notion that the LAT may be acting through antisense
or other RNA-mediated mechanisms to suppress nearby lytic phase
transcripts. Furthermore, the changes observed in the acetylation
status of the LAT enhancer indicate that the LAT enhancer is
both sensitive and responsive to reactivation signals. The increased
level of acetylation at the ICP0 promoter following deacetylation
of the LAT enhancer suggests that chromatin remodeling both
at the LAT locus and at the ICP0 promoter may be directly linked
during reactivation. It is therefore possible that the LAT enhancer
functions to recruit a novel histone-modifying complex that
helps establish and maintain the active expression of the LAT
during latency. During reactivation, the complex may quickly
respond to restructure the chromatin within the LAT region to
facilitate nearby lytic phase gene expression. Work aimed at
identifying such a complex is currently under way.

ACKNOWLEDGMENTS
This work was supported by grant AI48633 from the National Institutes
of Health and in part by the Investigators in Pathogenesis Award
from the Burroughs Wellcome Fund (to D.C.B.). N.J.K. and N.V.G.
received support from NIH training grant AI07110.
The authors thank P. McAnany for excellent technical assistance and J. Feller for helpful comments on the manuscript.

FOOTNOTES
* Corresponding author. Mailing address: Department of Molecular Genetics and Microbiology, Box 100266, University of Florida College of Medicine, Gainesville, FL 32610-0266. Phone: (352) 392-8520. Fax: (352) 392-3133. E-mail:
dbloom{at}ufl.edu.

A.L.A. and N.V.G. contributed equally to this work. 

REFERENCES
- 1 Berthomme, H., J. Lokensgard, L. Yang, T. Margolis, and L. T. Feldman. 2000. Evidence for a bidirectional element located downstream from the herpes simplex virus type 1 latency-associated promoter that increases its activity during latency. J. Virol. 74:3613-3622.[Abstract/Free Full Text]
- 2 Bloom, D. C., J. T. Hill, E. K. Wagner, L. F. Feldman, and J. G. Stevens. 1996. A 348-bp region in the latency-associated transcript facilitates herpes simplex virus type 1 reactivation. J. Virol. 70:2449-2459.[Abstract/Free Full Text]
- 3 Chen, S. H., M. F. Kramer, P. A. Schaffer, and D. M. Coen. 1997. A viral function represses accumulation of transcripts from productive-cycle genes in mouse ganglia latently infected with herpes simplex virus. J. Virol. 71:5878-5884.[Abstract/Free Full Text]
- 4 Deshmane, S. L., and N. W. Fraser. 1989. During latency, herpes simplex virus type 1 DNA is associated with nucleosomes in a chromatin structure. J. Virol. 63:943-947.[Abstract/Free Full Text]
- 5 Farrell, M. J., A. T. Dobson, and L. T. Feldman. 1991. Herpes simplex virus latency-associated transcript is a stable intron. Proc. Natl. Acad. Sci. USA 88:790-794.[Abstract/Free Full Text]
- 6 Hill, J. M., F. Sedarati, R. T. Javier, E. K. Wagner, and J. G. Stevens. 1990. Herpes simplex virus latent phase transcription facilitates in vivo reactivation. Virology 174:117-125.[CrossRef][Medline]
- 7 Jarman, R. G., J. M. Loutsch, G. B. Devi-Rao, M. E. Marquart, M. P. Banaszak, X. Zheng, J. M. Hill, E. K. Wagner, and D. C. Bloom. 2002. The region of the HSV-1 latency-associated transcript required for epinephrine-induced reactivation in the rabbit does not include the 2.0-kb intron. Virology 292:59-69.[CrossRef][Medline]
- 8 Kubat, N. J., A. L. Amelio, N. V. Giordani, and D. C. Bloom. 2004. The herpes simplex virus type 1 latency-associated transcript (LAT) enhancer/rcr is hyperacetylated during latency independently of LAT transcription. J. Virol. 78:12508-12518.[Abstract/Free Full Text]
- 9 Kubat, N. J., R. K. Tran, P. McAnany, and D. C. Bloom. 2004. Specific histone tail modification and not DNA methylation is a determinant of herpes simplex virus type 1 latent gene expression. J. Virol. 78:1139-1149.[Abstract/Free Full Text]
- 10 Leib, D. A., C. L. Bogard, V. M. Kosz, K. A. Hicks, D. M. Coen, D. M. Knipe, and P. A. Schaffer. 1989. A deletion mutant of the latency-associated transcript of herpes simplex virus type 1 reactivates from the latent state with reduced frequency. J. Virol. 63:2893-2900.[Abstract/Free Full Text]
- 11 Pesola, J. M., J. Zhu, D. M. Knipe, and D. M. Coen. 2005. Herpes simplex virus 1 immediate-early and early gene expression during reactivation from latency under conditions that prevent infectious virus production. J. Virol. 79:14516-14525.[Abstract/Free Full Text]
- 12 Rock, D. L., and N. W. Fraser. 1985. Latent herpes simplex virus type 1 DNA contains two copies of the virion DNA joint region. J. Virol. 55:849-852.[Abstract/Free Full Text]
- 13 Rock, D. L., A. B. Nesburn, H. Ghaisi, J. Ong, T. L. Lewis, J. R. Lokensgard, and S. L. Wechsler. 1987. Detection of latency-related viral RNAs in trigeminal ganglia of rabbits latently infected with herpes simplex virus. J. Virol. 61:3820-3826.[Abstract/Free Full Text]
- 14 Spivack, J. G., and N. W. Fraser. 1987. Detection of herpes simplex virus type 1 transcripts during latent infection in mice. J. Virol. 61:3841-3847.[Abstract/Free Full Text]
- 15 Stevens, J. G., and M. L. Cook. 1971. Latent herpes simplex virus in spinal ganglia of mice. Science 173:843-845.[Abstract/Free Full Text]
- 16 Stevens, J. G., E. K. Wagner, R. G. B. Devi, M. L. Cook, and L. T. Feldman. 1987. RNA complementary to a herpesvirus alpha gene mRNA is prominent in latently infected neurons. Science 235:1056-1059.[Abstract/Free Full Text]
- 17 Tal-Singer, R., T. M. Lasner, W. Podrzucki, A. Skokotas, J. J. Leary, S. L. Berger, and N. W. Fraser. 1997. Gene expression during reactivation of herpes simplex virus type 1 from latency in the peripheral nervous system is different from that during lytic infection of tissue cultures. J. Virol. 71:5268-5276.[Abstract/Free Full Text]
- 18 Wang, Q. Y., C. Zhou, K. E. Johnson, R. C. Colgrove, D. M. Coen, and D. M. Knipe. 2005. Herpesviral latency-associated transcript gene promotes assembly of heterochromatin on viral lytic-gene promoters in latent infection. Proc. Natl. Acad. Sci. USA 102:16055-16059.[Abstract/Free Full Text]
Journal of Virology, February 2006, p. 2063-2068, Vol. 80, No. 4
0022-538X/06/$08.00+0 doi:10.1128/JVI.80.4.2063-2068.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
This article has been cited by other articles:
-
Lacasse, J. J., Schang, L. M.
(2010). During Lytic Infections, Herpes Simplex Virus Type 1 DNA Is in Complexes with the Properties of Unstable Nucleosomes. J. Virol.
84: 1920-1933
[Abstract]
[Full Text]
-
Peng, H., Nogueira, M. L., Vogel, J. L., Kristie, T. M.
(2010). Transcriptional coactivator HCF-1 couples the histone chaperone Asf1b to HSV-1 DNA replication components. Proc. Natl. Acad. Sci. USA
107: 2461-2466
[Abstract]
[Full Text]
-
Whitlow, Z. W., Kristie, T. M.
(2009). Recruitment of the Transcriptional Coactivator HCF-1 to Viral Immediate-Early Promoters during Initiation of Reactivation from Latency of Herpes Simplex Virus Type 1. J. Virol.
83: 9591-9595
[Abstract]
[Full Text]
-
Kutluay, S. B., Triezenberg, S. J.
(2009). Regulation of Histone Deposition on the Herpes Simplex Virus Type 1 Genome during Lytic Infection. J. Virol.
83: 5835-5845
[Abstract]
[Full Text]
-
Liu, X.-f., Yan, S., Abecassis, M., Hummel, M.
(2008). Establishment of Murine Cytomegalovirus Latency In Vivo Is Associated with Changes in Histone Modifications and Recruitment of Transcriptional Repressors to the Major Immediate-Early Promoter. J. Virol.
82: 10922-10931
[Abstract]
[Full Text]
-
Kolb, G., Kristie, T. M.
(2008). Association of the Cellular Coactivator HCF-1 with the Golgi Apparatus in Sensory Neurons. J. Virol.
82: 9555-9563
[Abstract]
[Full Text]
-
Coleman, H. M., Connor, V., Cheng, Z. S. C., Grey, F., Preston, C. M., Efstathiou, S.
(2008). Histone modifications associated with herpes simplex virus type 1 genomes during quiescence and following ICP0-mediated de-repression. J. Gen. Virol.
89: 68-77
[Abstract]
[Full Text]
-
Neumann, D. M., Bhattacharjee, P. S., Giordani, N. V., Bloom, D. C., Hill, J. M.
(2007). In Vivo Changes in the Patterns of Chromatin Structure Associated with the Latent Herpes Simplex Virus Type 1 Genome in Mouse Trigeminal Ganglia Can Be Detected at Early Times after Butyrate Treatment. J. Virol.
81: 13248-13253
[Abstract]
[Full Text]
-
Preston, C. M.
(2007). Reactivation of Expression from Quiescent Herpes Simplex Virus Type 1 Genomes in the Absence of Immediate-Early Protein ICP0. J. Virol.
81: 11781-11789
[Abstract]
[Full Text]
-
Ozawa, S., Eda, H., Ishii, Y., Ban, F., Funabashi, T., Hata, S., Hayashi, K., Iga, H., Ikushima, T., Ishiko, H., Itagaki, T., Kawana, R., Kobayashi, S., Ogino, T., Sekizawa, T., Shimomura, Y., Shiota, H., Mori, R., Nakakita, T., Numazaki, Y., Ozaki, Y., Yamamoto, S., Yoshino, K., Yanagi, K.
(2007). The Herpes Simplex Virus Type 1 BgKL Variant, Unlike the BgOL Variant, Shows a Higher Association with Orolabial Infection than with Infections at Other Sites, Supporting the Variant-Dispersion-Replacement Hypothesis. J. Clin. Microbiol.
45: 2183-2190
[Abstract]
[Full Text]
-
Neumann, D. M., Bhattacharjee, P. S., Hill, J. M.
(2007). Sodium Butyrate: a Chemical Inducer of In Vivo Reactivation of Herpes Simplex Virus Type 1 in the Ocular Mouse Model. J. Virol.
81: 6106-6110
[Abstract]
[Full Text]
-
Alberter, B., Ensser, A.
(2007). Histone Modification Pattern of the T-Cellular Herpesvirus Saimiri Genome in Latency. J. Virol.
81: 2524-2530
[Abstract]
[Full Text]
-
Gussow, A. M., Giordani, N. V., Tran, R. K., Imai, Y., Kwiatkowski, D. L., Rall, G. F., Margolis, T. P., Bloom, D. C.
(2006). Tissue-Specific Splicing of the Herpes Simplex Virus Type 1 Latency-Associated Transcript (LAT) Intron in LAT Transgenic Mice. J. Virol.
80: 9414-9423
[Abstract]
[Full Text]