Progressive multifocal leukoencephalopathy is a fatal demyelinating disorder due to human polyomavirus JC infection in which there are viral inclusions in enlarged nuclei of infected oligodendrocytes. We report that the pathogenesis of this disease is associated with distinct subnuclear structures known as promyelocytic leukemia nuclear bodies (PML-NBs). Postmortem brain tissues from 5 patients with the disease were examined. Affected cells with enlarged nuclei contained distinct dot-like subnuclear PML-NBs that were immunopositive for PML protein and nuclear body protein Sp100. Major and minor viral capsid proteins and proliferating cell nuclear antigen, an essential component for DNA replication, colocalized with PML-NBs. By in situ hybridization, viral genomic DNA showed dot-like nuclear accumulation, and by electron microscopy, virus-like structures clustered in subnuclear domains, indicating that PML-NBs are the site of viral DNA replication and capsid assembly. Molecules involved in the ubiquitin proteosome pathway (i.e. ubiquitin and small ubiquitin-like modifier 1) did not accumulate in the nuclei with viral inclusions, indicating that cell degeneration may not be dependent on this pathway. When viral progeny production was advanced, PML-NBs were disrupted. These data suggest that: 1) PML-NBs allow for efficient viral propagation by providing scaffolds, 2) disruption of PML-NBs is independent of the ubiquitin-proteasome pathway, and 3) this disruption probably heralds oligodendrocyte degeneration and the resulting demyelination.
Progressive multifocal leukoencephalopathy is a fatal demyelinating disorder of the central nervous system due to human polyomavirus JC (JCV) infection. Oligodendrocytes in affected tissues display markedly enlarged nuclei in which JCV was identified in early electron microscopy studies as round and filamentous structures (1, 2). The virus has an icosahedral capsid composed of a major capsid protein, VP1, and minor capsid proteins, VP2/VP3; double-stranded circular DNA comprises its genome (3, 4). Based on electron microscopic analyses, when virions infect oligodendrocytes and first reach the nucleus, the nuclear volume increases. Then viral progeny appears in the vicinity of the inner nuclear membrane (5). Even after more than a quarter of a century, however, the molecular mechanisms that underlie the development of viral inclusions in oligodendrocytes remain unclear.
Promyelocytic leukemia nuclear bodies (PML-NBs; also known as PML oncogenic domains or nuclear domain 10) are ubiquitous subnuclear structures in the interphase nucleus (6-8). Promyelocytic leukemia nuclear bodies were first identified in acute promyelocytic leukemia in which PML-NBs are disrupted due to the synthesis of fusion proteins of PML/retinoic acid receptor-α in association with a t(15;17) chromosomal translocation (9, 10). Promyelocytic leukemia nuclear bodies in proliferating cells typically appear as 10 to 30 small dots with diameters of 0.2 to 1 μm. The numbers and sizes of PML-NBs vary throughout the cell cycle; the smallest average number is seen in G0; the numbers slowly increase during progression to G1; the highest number is observed in the S phase (11). More than 50 proteins have been reported to localize either transiently or consistently to PML-NBs. Constitutive components of PML-NBs include PML protein and the nuclear body Sp100 protein. Promyelocytic leukemia protein is a tumor suppressor (12-17), and Sp100 influences transcription and chromatin dynamics (18, 19).
Using COS-7 cells, we found that the JC virus capsid proteins accumulate and are efficiently assembled into virus-like structures in PML-NBs (20). Moreover, in addition to JCV, other DNA viruses cause PML-NB-associated degeneration or transformation of host cells (21, 22). For example, herpes simplex virus type 1, human cytomegalovirus, and Epstein-Barr virus all disrupt PML-NBs so that they can replicate efficiently in early stages of the viral replication cycle (23-25). By contrast, human papillomavirus type 33 reorganizes components of PML-NBs, using these bodies as a site for efficient progeny production during the late stage of replication (26, 27). Promyelocytic leukemia nuclear bodies are also related to multiple histologic types of solid cancers, including prostate, thyroid, colon, breast, lung, and brain (28-33); decreased expression of PML protein has been reported in most of these cancer cell types. In addition, PML-NBs are reorganized in neurodegenerative disorders such as polyglutamine diseases. In these diseases, the disease-related proteins such as ataxin or huntingtin accumulate at PML-NBs with modification of small ubiquitin-like modifier 1 (SUMO-1). This results in the formation of ubiquitin-positive neuronal intranuclear inclusions, and involvement of the ubiquitin-proteasome pathway has therefore been suggested as the mechanism of cell degeneration (34-42). Thus, PML-NBs are involved in the pathogenesis of a wide variety of human diseases, but the role of PML-NBs in progressive multifocal leukoencephalopathy, a disorder caused by infection of the highly oncogenic DNA virus JCV, is not known.
In this study, we investigated the ways that JCV interacts with PML-NBs in the human brain in cases of progressive multifocal leukoencephalopathy. We characterized the expression of PML-NB-related proteins and their association with JCV capsid proteins in different stages of lesion development. The findings indicate that JCV interactions with PML-NBs in oligodendrocytes with enlarged nuclei are associated with degeneration of these cells and demyelination.
Materials and Methods
Five autopsied cases (3 men, 2 women) of progressive multifocal leukoencephalopathy were studied. The mean age at death was 60 years (range, 35-85 years). Underlying diseases included AIDS (n = 2) and leukemia (n = 2); the remaining patient was an elderly woman without any associated underlying diseases.
Rabbit polyclonal antibody against the potential BC loop structure of VP1 (anti-VP1BC antibody) has been described previously (4). VP1-HI antibody was generously provided by Prof. Nagashima of Hokkaido University, Japan. Rabbit polyclonal antibody against VP2/VP3 was prepared against the C-terminal sequence RKEGPRASSKTSYKR as previously described (20). Antibodies to the following proteins were purchased: PML protein (MBL, Nagoya, Japan); Sp100 protein (Chemicon, Temecula, CA); proliferating cell nuclear antigen (PCNA) (Novocastra, Newcastle, UK); SUMO-1 (Zymed, San Francisco, CA); and ubiquitin (Santa Cruz Biotechnology, Santa Cruz, CA).
Histology and Immunohistochemistry
Formalin-fixed paraffin-embedded sections of human brain tissue were stained using hematoxylin and eosin and Luxol fast blue. For immunostaining, 5-μm-thick sections were deparaffinized in xylene then rehydrated in 90%, 70%, and 50% ethanol. For antigen retrieval, the sections were autoclaved in buffered citrate at 120°C for 10 minutes. Primary antibodies were incubated at 4°C for approximately 12 hours. The sections were then washed and incubated using biotinylated secondary antibodies, followed by avidin-biotin-peroxidase complex from a Vectastain avidin-biotin-peroxidase complex kit (Vector Laboratories, Burlingame, CA). Diaminobenzidine was used as the chromogen. After immunostaining, the sections were counterstained with hematoxylin.
After deparaffinization, sections were rinsed in phosphate-buffered saline (PBS) containing 0.05% Tween-20 then blocked in PBS containing 5% normal goat serum at room temperature for 30 minutes. Sections were then incubated with primary antibodies for 1 hour, washed in PBS containing 0.05% Tween-20, then further incubated using the appropriate secondary antibodies conjugated with either Alexa Fluor 488-conjugated anti-rabbit immunoglobulin G (IgG) or Alexa Fluor 568-conjugated anti-rabbit IgG (Molecular Probes, Invitrogen, Carlsbad, CA). After 3 washes in PBS, samples were mounted in VectaShield (Vector Laboratories). Fluorescent images were captured using a TCS-SP confocal laser microscope (Leica, Heidelberg, Germany).
In Situ Hybridization
Paraffin-embedded tissue sections were deparaffinized and rehydrated as above. The sections were pretreated by autoclave heating in buffered citrate at 120°C for 10 minutes, followed by incubation in 0.01% pepsin in 0.2 N HCl for 20 minutes at 26°C. The sections were then dehydrated through an increasing ethanol series and air-dried for 2 days. The appropriate probe mix 1 μg/ml was applied to the tissue section and then covered with a coverslip. The target DNA was denatured at 90°C for 5 minutes and incubated at 45°C for 60 minutes, 40°C for 60 minutes, and 35°C for more than 6 hours. Posthybridization washes were performed according to the procedures for human papillomavirus in use of GenPoint System (Dako, Carpinteria, CA). The sections were treated with 0.3% H2O2 in methanol at 26°C, and diaminobenzidine was used as the chromogen. The sections were then counterstained with hematoxylin. The single-strand DNA probe was targeted to the VP2 coding regions 5′-CTCCAGTAATTACAGCATATGT TTCAGGAGTAAGGCCTATTGCAGCTATAGCCTCAGAGGTACTTGTAAT-3′.
Transfection of COS-7 Cells and Immunocytochemistry
COS-7 cells were incubated at 37°C under 5% CO2 in Dulbecco's modified Eagle's minimum essential medium supplemented with 10% fetal bovine serum. The cells were transfected with the AVP231-SRα expression vector as described previously (4, 20, 43, 44) and then harvested 3 days after transfection. For immunofluorescence, transfected cells were fixed in 4% paraformaldehyde in PBS for 15 minutes at room temperature then permeabilized with 0.5% Triton X-100 in PBS for 20 minutes. Cells were subsequently rinsed in PBS containing 0.05% Tween-20 then blocked with PBS containing 5% normal goat serum at room temperature for 30 minutes. Lastly, the cells were incubated with antibodies using the same methods as for formalin-fixed paraffin-embedded tissues.
COS-7 cells were transfected with AVP231-SRα or VP231-SRα as described previously (4, 20, 43, 44) then harvested 3 days after transfection. The cells were homogenized in sodium dodecyl sulfate sample buffer and reduced by boiling at 95°C for 5 minutes. The protein was separated by electrophoresis on 12.5% sodium dodecyl sulfate-polyacrylamide gels and transferred to a nitrocellulose membrane in 50 mmol/L Tris buffer containing 145 mmol/L glycine. The membrane was blocked for 60 minutes at room temperature with 5% skim milk and then reacted with the anti-VP2/VP3 primary antibody. Peroxidase-labeled anti-rabbit IgG was used as the secondary antibody. The Western Lightning Chemiluminescence Reagent Plus Kit (PerkinElmer LAS, Inc., Boston, MA) was used for detection.
For conventional electron microscopy, brain tissues were fixed using 2.5% glutaraldehyde and 1.0% paraformaldehyde in 0.1 mol/L phosphate buffer (pH 7.3) and postfixed in 1.0% osmium tetroxide. After dehydration through a graded ethanol series, 50%, 70%, 80%, 90%, 95%, and 100% 3 times each, the tissues were embedded in epoxy resin. Ultrathin sections were prepared, stained using uranyl acetate and lead citrate, and examined using an H-7100 electron microscope (Hitachi, Tokyo, Japan). For immunoelectron microscopy, transfected COS-7 cells were fixed at 72 hours posttransfection in 4% paraformaldehyde in 0.1 mol/L phosphate buffer (pH 7.4) for 15 minutes. After dehydration through a graded ethanol series as previously described, the cells were embedded in LR White resin (Structure Probe, West Chester, PA). Ultrathin sections were prepared and mounted on nickel grids. After incubation with 10% normal goat serum for 10 minutes, sections were incubated overnight at 4°C with anti-VP1HI antibody. After washing with PBS, sections were incubated with goat anti-rabbit IgG conjugated to 15-nm gold particles (1:30; British BioCell International, Cardiff, UK) for 30 minutes at room temperature. Sections were then stained with uranyl acetate and examined by electron microscopy.
Dot-Like Subnuclear Structures (PML-NBs) Appear in Cells With Enlarging Nuclei
There were multiple demyelinated lesions in brains of patients with progressive multifocal leukoencephalopathy predominantly located in the cerebral hemispheric white matter (Fig. 1A). Numerous abnormal cells were apparent, most frequently at the periphery of demyelinated lesions (Fig. 1B). At the corticomedullary borders (i.e. areas with relatively intact myelin), the myelin sheaths were stained with Luxol fast blue (*1 in Fig. 1A, B). Cells with small but slightly enlarged nuclei were scattered in these areas (Fig. 1C). These cells usually displayed bright nuclei with marginated chromatin, which included small dots or fine granular structures stained with hematoxylin (Fig. 1D). The appearance of these cells was distinct from that of normal oligodendrocytes with condensed chromatin. In partially demyelinated areas, only degenerated myelin debris stained with Luxol fast blue, and myelin fiber sheaths were no longer seen (*2 in Fig. 1A, B). Numerous cells in these areas had nuclei that were approximately 3- or 4-fold larger than those of normal oligodendrocytes (Fig. 1E). Abnormal cells exhibited 2 distinct intranuclear staining patterns. Some showed amphophilic staining throughout the entire nucleus, whereas other cells showed bright nucleoplasm with marginated chromatin in which frequent dot-shaped structures that were well stained with hematoxylin were present at the nuclear periphery (arrows in Fig. 1F). In the completely demyelinated lesions that lacked staining with Luxol fast blue (*3 in Fig. 1A, B), the density of cells with enlarged nuclei was decreased, and foamy macrophages appeared.
Accumulation of JCV Capsid Proteins in PML-NBs
Immunolocalization of JCV-related epitopes was examined using antibodies against viral capsid proteins VP2/VP3 (Fig. 2). Because expression of VP2/VP3 has previously been reported only in transfected cell lines, this is the first demonstration of VP2/VP3 expression in human progressive multifocal leukoencephalopathy. At the corticomedullary borders (i.e. areas with relatively intact myelin), many small, slightly enlarged oligodendrocyte-like nuclei lacked immunoreactivity for viral capsid proteins (Fig. 2A, B). As in hematoxylin and eosin stains, cells with enlarged nuclei in neighboring regions with mild demyelination displayed 2 distinct staining patterns. Some showed accumulation of capsid proteins in the dot-like structures along the inner periphery of the bright nucleoplasm (Fig. 2A, B), and others displayed intense and diffuse staining for capsid proteins throughout the entire nucleus; in these cells, very little dot-like staining was seen (Fig. 2A, B).
The specificity of the VP2/VP3 antibody was confirmed by Western blotting using cultured cells expressing JCV capsid proteins. VP3 of approximately 26 kD was clearly detected, but the least capsid protein, VP2 of approximately 37 kD, was less than the detection threshold (Fig. 2C).
The dot-like subnuclear structures were identifiable as PML-NBs because they expressed both PML protein (Fig. 2D) and Sp100 protein (Fig. 2D). Frequently, the enlarged nuclei contained approximately 2 to 5 PML-NBs that were up to 2.0 μm in diameter. In contrast, normal oligodendrocyte-like nuclei rarely showed immunoreactivity for either PML or Sp100 proteins.
PML-NBs Can Be Sites of Viral Progeny Production
When double staining was performed for JCV major capsid protein VP1 and PML protein, some cells showed colocalization of VP1 and PML protein in dot-like subnuclear structures, indicating VP1 localization in the PML-NBs (Fig. 3A). Proliferating cell nuclear antigen, an essential component of DNA replication during S phase, also showed dot-like staining in some enlarged nuclei and was colocalized with PML protein (Fig. 3B). Both VP1-positive and PCNA-positive PML-NBs were frequently seen along the inner nuclear periphery.
The presence and localization of intranuclear viral DNA were also examined by in situ hybridization using a DNA probe complementary to the viral capsid proteins. The JCV DNA was detected as nuclear dot-like signals in enlarged nuclei of infected cells, indicating that replication of viral DNA molecules occurs at distinct subnuclear domains. Although JCV DNA signals were hardly seen at the corticomedullary borders (intact myelin), in areas with mild demyelination, large dot-like DNA signals were roughly or densely aligned in the nuclei (Fig. 3C).
By electron microscopy, JCV virions were seen as round or filamentous structures in enlarged nuclei. Although intranuclear amounts and distributions of virions varied in individual cells, many cells contained virions at the periphery, rather than in the center of the nucleus (Fig. 3D). We previously established an expression system to produce virus-like structures in COS-7 cells (4, 44), and in these cells, colloidal gold particles representing the VP1 epitope were similarly clustered along the inner nuclear periphery and clusters of numerous virus-like particles with both round and filamentous structures were evident (Fig. 3D). Therefore, the intranuclear distribution was quite similar to that of the viruses observed in human brain tissue.
These observations support the idea that the PML-NBs are the sites of JCV progeny production, and that in addition to capsid assembly, viral DNA replication can occur in the PML-NBs. Promyelocytic leukemia nuclear bodies thus seem to provide a scaffold for efficient viral replication, and the cells in which the virus particles are propagated seem to be in an S-phase-like state with expression of PCNA.
Distribution of the PML-NB-Related Proteins SUMO-1 and Ubiquitin
Promyelocytic leukemia nuclear bodies are known to be sites of protein degradation. In polyglutamine diseases and in other DNA virus infections, proteins modified with SUMO-1 (SUMOylation) are degraded through a ubiquitin-proteasome pathway. To investigate cell degeneration in JCV-infected cells, we next examined expression of SUMO-1 and ubiquitin. At corticomedullary borders, immunoreactivity for SUMO-1 was apparent in the nuclei of cortical neurons but not in oligodendrocyte-like nuclei (Fig. 4A). In the white matter with mild demyelination, some cells showed faint dot-like staining in enlarged nuclei, but these were relatively sparse (Fig. 4A). Enlarged nuclei with full basophilic inclusions were minimally immunoreactive for SUMO-1 (Fig. 4A).
Ubiquitin distribution also varied in different cells. At the corticomedullary borders, slightly enlarged oligodendrocyte-like nuclei showed dot-shaped immunostaining for ubiquitin. Most neurons exhibited diffuse cytoplasmic staining and/or nuclear dot-like staining (Fig. 4B). In mildly demyelinated white matter, only small oligodendrocyte-like nuclei were immunoreactive for ubiquitin, but there was none in markedly enlarged nuclei (Fig. 4B). Interestingly, ubiquitin immunoreactivity was present in the cytoplasm in cells containing viral inclusions throughout the nucleus (Fig. 4B).
These observations indicate that the localizations of SUMO-1 and ubiquitin are variable. Accumulation of SUMO-1 or ubiquitin in nuclei was not observed in cells with fully developed viral inclusions, suggesting that the degradation of JCV-infected oligodendrocytes is not tightly linked to the ubiquitin-proteasome pathway.
PML-NB Structures Are Disrupted in Fully Demyelinated Lesions
Finally, the morphologic features of PML-NBs were examined in association with degrees of JCV-induced demyelination. Brain tissues were double-stained for JCV VP1 and PML protein to assess cell degeneration after production of progeny virions. Promyelocytic leukemia protein immunoreactivity was seen over a relatively broad area. Promyelocytic leukemia protein and VP1 were colocalized to PML-NBs in both partially demyelinated areas (Fig. 5A) and in areas with more marked demyelination (Fig. 5B). In the center of large demyelinated lesions, infected cells were severely degraded. In these, VP1 and PML protein barely showed dot-like staining and instead exhibited more marginated staining in degenerated nuclei (Fig. 5C). In severely degenerated lesions, VP1 and PML stained rather nonspecifically in cells that seemed to be close to death (Fig. 5D). These observations indicate that PML-NBs are progressively disrupted with viral infection in a process associated with the oligodendrocyte degeneration and consequent demyelination.
This is the first comprehensive study to demonstrate targeted localization of JCV capsid proteins to PML-NBs in oligodendrocyte-like cells within and around demyelinated foci in human progressive multifocal leukoencephalopathy. Because viral DNA accumulate in enlarged nuclei of these cells, it is likely that PML-NBs provide a structural support for JCV replication in these cells. Disruption of PML-NBs occurs when there is oligodendrocyte degeneration and subsequent demyelination. These observations indicate that PML-NBs in progressive multifocal leukoencephalopathy function in a manner partly similar to other viral infections and, to some extent, cancer, but that their function is somehow distinct from polyglutamine diseases.
Human polyomavirus JC is unique because both major capsid protein VP1 and minor capsid proteins VP2/VP3 cooperatively target PML-NBs for efficient production of progeny virions (20). Expression of PCNA in the inner nuclear periphery suggests that these intranuclear viral inclusions are formed in an S-phase-like state of the cell cycle. Some cells exhibit dot-like accumulations of PCNA at the inner nuclear periphery (Fig. 3). This is similar to dividing cells in S phase in which dot-like accumulations of PCNA align along the nuclear periphery (45) and are associated with PML-NBs (46). Moreover, PML-NBs have been reported to be sites of DNA replication in several DNA viruses (47-51). Thus, viral replication is tightly associated with the host cell machinery, and viral DNA replication and capsid assembly seem to occur simultaneously at PML-NBs. In addition to PCNA, JCV-infected oligodendrocytes show very high expression of MIB-1 and are also positive for cyclins A and B1, which are expressed during S-G2-M phases (52-55). Therefore, the cell cycle of the infected oligodendrocytes likely is highly activated until formation of viral inclusions and provides a cellular environment that promotes viral infection. It remains unclear, however, why activated oligodendrocytes are degraded, rather than undergo mitotic division.
The mechanisms of oligodendrocyte degeneration after the production of viral progeny are not clear. Promyelocytic leukemia nuclear bodies are also known to be the site of regulated protein degradation through a ubiquitin-proteasome pathway. Herpes simplex virus and human cytomegalovirus abrogate SUMOylated PML through a proteasome-dependent pathway, resulting in the disruption of PML-NBs (56-58). Similar mechanisms may explain neurodegeneration in polyglutamine diseases with ubiquitin-positive neuronal intranuclear inclusions (34, 37). In JCV infection, however, SUMO-1 and ubiquitin were not found to accumulate in the nucleus with full inclusions (Fig. 4). Although PML-NBs of JCV-infected oligodendrocytes were disrupted along with development of viral inclusions and resulting demyelination (Fig. 5), the mechanisms of PML-NB disruption seem to be unrelated to the ubiquitin-proteasome pathway. We currently speculate that JCV-infected cells would burst in an M-phase-like state. In normal-dividing cells, PML-NBs disappear during M phase. Many JCV-infected cells positive for cyclin A showed degraded nuclear membrane with scant and unclear cytoplasm, as if they were bursting, rather than undergoing mitosis (data not shown).
In conclusion, PML-NBs may play important roles in progressive multifocal leukoencephalopathy. Promyelocytic leukemia nuclear bodies likely provide a scaffold for the replication of viral progenies, providing a cancer-like cellular environment, and these are finally disrupted after the development of viral inclusions. It remains unclear as to what determines whether a cell is to undergo degeneration or transformation. Human polyomavirus JC is highly oncogenic and is known to induce brain tumors in various animal models (59-61); the presence of the JCV genome has also been reported in different types of human cancers (62-65). It is hoped that future investigations will clarify the dynamic pathogenic roles of PML-NBs, leading to either cell degeneration or transformation, in association with JCV.
The authors thank Michiru Umino, Akiko Kitazawa, Ayumi Sumiishi, Kaoruko Kojima, and Kazuko Nakayama of the Department of Pathology at Kyorin University School of Medicine for technical assistance.