Splice variants of the nuclear dot-associated Sp100 protein contain homologies to HMG-1 and a human nuclear phosphoprotein-box motif

PML Body Component Sp100A Is a Cytosolic Responder to IFN and Activator of Antiviral ISGs

Promyelocytic leukemia protein (PML) bodies are implicated in one of the key pathways in the establishment of antiviral status in response to interferon (IFN), yet the molecular mechanisms bridging the cross talk remain elusive. Herein, we report that a major constitutive component of the PML body, Sp100A, is ubiquitously located in the cytosol of various cell types and is an immediate responder to multiple extracellular stimuli, including virus infection, IFN, epidermal growth factor (EGF), glial cell-derived nerve factor (GDNF), etc., signaling through the phosphatidylinositol 3-kinase (PI3K) pathway. IFN-β induces phosphorylation of Sp100A on Ser¹⁸⁸, which fortifies the binding of Sp100A to pyruvate kinase 2 (PKM2) and facilitates its nuclear importation through the extracellular signal-regulated kinase 1/2 (ERK1/2)-PKM2-PIN1-importin axes. Blocking PI3K pathway signaling or interference with the ERK1/2-PKM2-PIN1-importin axes independently hampers nuclear translocation of Sp100A in response to IFN, reflecting a dual-regulation mechanism governing this event. In the nucleus, Sp100A is enriched in the promoter regions of essential antiviral interferon-stimulated genes (ISGs), such as those coding for IFI16, OAS2, and RIG-I, and activates their transcription. Importantly, nuclear importation of Sp100A, but not accumulation of a mutant Sp100A that failed to respond to IFN, during infection potently enhanced transcription of these antiviral ISGs and restricted virus propagation. These findings depict a novel IFN response mechanism by PML bodies in the cytosol and shed light on the complex sensing-regulatory network of PML bodies. IMPORTANCE PML bodies sit at the center stage of various important biological processes; however, the signal transduction networks of these macromolecular protein complexes remain enigmatic. The present study illustrates, in detail and for the first time, the course of signal receiving, processing, and implementation by PML bodies in response to IFN and virus infection. It shows that PML body constitutive component Sp100A was phosphorylated on Ser¹⁸⁸ by IFN signaling through the PI3K pathway in the cytosol, cotranslocated into the nucleus with PKM2, enriched on the promoter regions of essential antiviral ISGs such as those coding for IFI16, RIG-I, OAS2, etc., and mediating their transcriptional activation.

PML Body Component Sp100A Restricts Wild-Type Herpes Simplex Virus 1 Infection

Sp100 (speckled protein 100 kDa) is a constituent component of nuclear structure PML (promyelocytic leukemia) bodies, playing important roles in mediating intrinsic and innate immunity. The Sp100 gene encodes four isoforms with distinct roles in the transcriptional regulation of both cellular and viral genes. Since Sp100 is a primary intranuclear target of infected-cell protein 0 (ICP0), an immediate early E3 ligase encoded by herpes simplex virus 1 (HSV-1), previous investigations attempting to analyze the functions of individual Sp100 variants during HSV-1 infection mostly avoided using a wild-type virus. Therefore, the role of Sp100 under natural infection by HSV-1 remains to be clarified. Here, we reappraised the antiviral capacity of four Sp100 isoforms during infection by a nonmutated HSV-1, examined the molecular behavior of the Sp100 protein in detail, and revealed the following intriguing observations. First, Sp100 isoform A (Sp100A) inhibited wild-type HSV-1 propagation in HEp-2, Sp100-/-, and PML-/- cells. Second, endogenous Sp100 is located in both the nucleus and the cytoplasm. During HSV-1 infection, the nuclear Sp100 level decreased drastically upon the detection of ICP0 in the same subcellular compartment, but cytosolic Sp100 remained stable. Third, transfected Sp100A showed subcellular localizations similar to those of endogenous Sp100 and matched the protein size of endogenous cytosolic Sp100. Fourth, HSV-1 infection induced increased secretion of endogenous Sp100 and ectopically expressed Sp100A, which copurified with extracellular vesicles (EVs) but not infectious virions. Fifth, the Sp100A level in secreting cells positively correlated with its level in EVs, and EV-associated Sp100A restricted HSV-1 in recipient cells. IMPORTANCE Previous studies show that the PML body component Sp100 protein is immediately targeted by ICP0 of HSV-1 in the nucleus during productive infection. Therefore, extensive studies investigating the interplay of Sp100 isoforms with HSV-1 were conducted using a mutant virus lacking ICP0 or in the absence of infection. The role of Sp100 variants during natural HSV-1 infection remains blurry. Here, we report that Sp100A potently and independently inhibited wild-type HSV-1 and that during HSV-1 infection, cytosolic Sp100 remained stable and was increasingly secreted into the extracellular space, in association with EVs. Furthermore, the Sp100A level in secreting cells positively correlated with its level in EVs and the anti-HSV-1 potency of these EVs in recipient cells. In summary, this study implies an active antiviral role of Sp100A during wild-type HSV-1 infection and reveals a novel mechanism of Sp100A to restrict HSV-1 through extracellular communications.

Speckled 100 kDa gene in pigs: Alternative splicing, subcellular localization, and response to interferon-α stimulation

Speckled 100 kDa (Sp100) plays an important role in the antiviral immune response, however, little is known about porcine Sp100. In this study, porcine Sp100 was cloned and its response to interferon (IFN) α was identified. We obtained the cDNA (V1) of the gene, SP100, and seven alternative splicing variants (V2-8). Isoform V1 encoded a 386 amino acid protein and contained a homogeneously-staining region (HSR) domain. Isoforms V3, 4, 6 and 7 were deletion/insertion variants and contained HSR domain as V1. The splicing of porcine SP100 was very complicated and many transcripts existed as revealed by cloning and minigene analyses. Using GFP-fusion constructs isoforms V1, 3, 4, 6 and 7 were localized to nucleus and the nuclear localization signal was identified as PSNRKRR at positions 331-337 of V1. Porcine SP100 was unevenly distributed in all tissues studied and differentially expressed between pigs with different disease-resistance/susceptibilities. Porcine SP100 was strongly increased by IFNα due to the existence of an IFN-stimulated response element in the promoter. A single nucleotide - 70A > C polymorphism enhanced promoter activity. The results provided the basis for determining the role of Sp100 in antiviral responses and may assist in breeding pigs with high disease-resistance.

Viral DNA Binding Protein SUMOylation Promotes PML Nuclear Body Localization Next to Viral Replication Centers

Human adenoviruses (HAdVs) have developed mechanisms to manipulate cellular antiviral measures to ensure proper DNA replication, with detailed processes far from being understood. Host cells repress incoming viral genomes through a network of transcriptional regulators that normally control cellular homeostasis. The nuclear domains involved are promyelocytic leukemia protein nuclear bodies (PML-NBs), interferon-inducible, dot-like nuclear structures and hot spots of SUMO posttranslational modification (PTM). In HAdV-infected cells, such SUMO factories are found in close proximity to newly established viral replication centers (RCs) marked by the adenoviral DNA binding protein (DBP) E2A. Here, we show that E2A is a novel target of host SUMOylation, leading to PTMs supporting E2A function in promoting productive infection. Our data show that SUMOylated E2A interacts with PML. Decreasing SUMO-E2A protein levels by generating HAdV variants mutated in the three main SUMO conjugation motifs (SCMs) led to lower numbers of viral RCs and PML-NBs, and these two structures were no longer next to each other. Our data further indicate that SUMOylated E2A binds the host transcription factor Sp100A, promoting HAdV gene expression, and represents the molecular bridge between PML tracks and adjacent viral RCs. Consequently, E2A SCM mutations repressed late viral gene expression and progeny production. These data highlight a novel mechanism used by the virus to benefit from host antiviral responses by exploiting the cellular SUMO conjugation machinery.IMPORTANCE PML nuclear bodies (PML-NBs) are implicated in general antiviral defense based on recruiting host restriction factors; however, it is not understood so far why viruses would establish viral replication centers (RCs) juxtaposed to such "antiviral" compartments. To understand this enigma, we investigate the cross talk between PML-NB components and viral RCs to find the missing link connecting both compartments to promote efficient viral replication and gene expression. Taken together, the current concept is more intricate than originally believed, since viruses apparently take advantage of several specific PML-NB-associated proteins to promote productive infection. Simultaneously, they efficiently inhibit antiviral measures to maintain the viral infectious program. Our data provide evidence that SUMOylation of the viral RC marker protein E2A represents the basis of this virus-host interface and regulates various downstream events to support HAdV productive infection. These results are the basis of our current attempts to generate and screen for specific E2A SUMOylation inhibitors to constitute novel therapeutic approaches to limit and prevent HAdV-mediated diseases and mortality of immunosuppressed patients.

Sp100 colocalizes with HPV replication foci and restricts the productive stage of the infectious cycle

We have shown previously that Sp100 (a component of the ND10 nuclear body) represses transcription, replication and establishment of incoming human papillomavirus (HPV) DNA in the early stages of infection. In this follow up study, we show that Sp100 does not substantially regulate viral infection in the maintenance phase, however at late stages of infection Sp100 interacts with amplifying viral genomes to repress viral processes. We find that Sp100 localizes to HPV16 replication foci generated in primary keratinocytes, to HPV31 replication foci that form in differentiated cells, and to HPV16 replication foci in CIN 1 cervical biopsies. To analyze this further, Sp100 was down regulated by siRNA treatment of differentiating HPV31 containing cells and levels of viral transcription and replication were assessed. This revealed that Sp100 represses viral transcription and replication in differentiated cells. Analysis of Sp100 binding to viral chromatin showed that Sp100 bound across the viral genome, and that binding increased at late stages of infection. Therefore, Sp100 represses the HPV life cycle at both early and late stages of infection.

MORC3, a Component of PML Nuclear Bodies, Has a Role in Restricting Herpes Simplex Virus 1 and Human Cytomegalovirus

We previously reported that MORC3, a protein associated with promyelocytic leukemia nuclear bodies (PML NBs), is a target of herpes simplex virus 1 (HSV-1) ICP0-mediated degradation (E. Sloan, et al., PLoS Pathog 11:e1005059, 2015, http://dx.doi.org/10.1371/journal.ppat.1005059). Since it is well known that certain other components of the PML NB complex play an important role during an intrinsic immune response to HSV-1 and are also degraded or inactivated by ICP0, here we further investigate the role of MORC3 during HSV-1 infection. We demonstrate that MORC3 has antiviral activity during HSV-1 infection and that this antiviral role is counteracted by ICP0. In addition, MORC3's antiviral role extends to wild-type (wt) human cytomegalovirus (HCMV) infection, as its plaque-forming efficiency increased in MORC3-depleted cells. We found that MORC3 is recruited to sites associated with HSV-1 genomes after their entry into the nucleus of an infected cell, and in wt infections this is followed by its association with ICP0 foci prior to its degradation. The RING finger domain of ICP0 was required for degradation of MORC3, and we confirmed that no other HSV-1 protein is required for the loss of MORC3. We also found that MORC3 is required for fully efficient recruitment of PML, Sp100, hDaxx, and γH2AX to sites associated with HSV-1 genomes entering the host cell nucleus. This study further unravels the intricate ways in which HSV-1 has evolved to counteract the host immune response and reveals a novel function for MORC3 during the host intrinsic immune response.

IMPORTANCE

Herpesviruses have devised ways to manipulate the host intrinsic immune response to promote their own survival and persistence within the human population. One way in which this is achieved is through degradation or functional inactivation of PML NB proteins, which are recruited to viral genomes in order to repress viral transcription. Because MORC3 associates with PML NBs in uninfected cells and is a target for HSV-1-mediated degradation, we investigated the role of MORC3 during HSV-1 infection. We found that MORC3 is also recruited to viral HSV-1 genomes, and importantly it contributes to the fully efficient recruitment of PML, hDaxx, Sp100, and γH2AX to these sites. Depletion of MORC3 resulted in an increase in ICP0-null HSV-1 and wt HCMV replication and plaque formation; therefore, this study reveals that MORC3 is an antiviral factor which plays an important role during HSV-1 and HCMV infection.

Sp100A is a tumor suppressor that activates p53-dependent transcription and counteracts E1A/E1B-55K-mediated transformation

Human adenoviruses (HAdV) are used as a model system to investigate tumorigenic processes in mammalian cells where the viral oncoproteins E1A and E1B-55K are absolutely required for oncogenic transformation, because they simultaneously accelerate cell cycle progression and inhibit tumor suppressor proteins such as p53, although the underlying mechanism is still not understood in detail. In our present study, we provide evidence that E1B-55K binding to the PML-NB component Sp100A apparently has an essential role in regulating adenovirus-mediated transformation processes. Specifically, when this E1B-55K/Sp100A complex recruits p53, Sp100A-induced activation of p53 transcriptional activity is effectively abolished. Hence, Sp100A exhibits tumor-suppressive activity, not only by stabilizing p53 transactivation but also by depressing E1A/E1B-55K-mediated transformation. E1B-55K counteracts this suppressive activity, inducing Sp100A SUMOylation and sequestering the modified cellular factor into the insoluble matrix of the nucleus or into cytoplasmic inclusions. These observations provide novel insights into how E1B-55K modulates cellular determinants to maintain growth-promoting activity during oncogenic processes and lytic infection.

Molecular characterization of Sp110 gene in pigs

Speckled 110 kDa (Sp110) plays an important role in infectious diseases, as revealed by studies in humans. However, little is known regarding porcine Sp110. To elucidate its potential role in porcine resistance to viral diseases, here, the complete coding sequence of porcine Sp110 gene and its 26 alternatively spliced isoforms were isolated using reverse transcription (RT)-polymerase chain reaction (PCR), and another seven splicing patterns were obtained using a minigene construct. Subcellular distribution of 11 representative isoforms was characterized in PK-15 cells transiently transfected with their respective GFP fusion constructs, and only isoforms (R and V) bearing all functional domains were localized in nucleus, indicating all the other isoforms lose normal functions of Sp110 owing to alternative splicing. Real-time quantitative PCR and competitive RT-PCR showed that both isoforms R and V had similar tissue expression profile, half-life and response to poly(I:C), a synthetic analog of viral double-stranded RNA, while the longer one (isoform R) was transcribed at a higher level. The results indicated that porcine Sp110 has a role in viral infection and that isoform R is the dominant active form. Overall the data provide potential resource for molecular breeding of pig resistant to diseases and contributes to breeding pigs resistant to viral infection.

Interferon Kappa Inhibits Human Papillomavirus 31 Transcription by Inducing Sp100 Proteins

High-risk human papillomaviruses (hr-HPV) establish persistent infections in keratinocytes, which can lead to cancer of the anogenital tract. Interferons (IFNs) are a family of secreted cytokines that induce IFN-stimulated genes (ISGs), many of which display antiviral activities. Transcriptome studies have indicated that established hr-HPV-positive cell lines display a reduced expression of ISGs, which correlates with decreased levels of interferon kappa (IFN-κ), a type I IFN constitutively expressed in keratinocytes. Prior studies have also suggested that IFN-β has anti-hr-HPV activity but the underlying mechanisms are not well understood. The downregulation of IFN-κ by hr-HPV raises the possibility that IFN-κ has anti-HPV activity. Using doxycycline-inducible IFN-κ expression in CIN612-9E cells, which maintain extrachromosomally replicating HPV31 genomes, we demonstrated that IFN-κ inhibits the growth of these cells and reduces viral transcription and replication. Interestingly, the initiation of viral early transcription was already inhibited at 4 to 6 h after IFN-κ expression. This was also observed with recombinant IFN-β, suggesting a common mechanism of IFNs. Transcriptome sequencing (RNA-seq) analysis identified 1,367 IFN-κ-regulated genes, of which 221 were modulated >2-fold. The majority of those (71%) matched known ISGs, confirming that IFN-κ acts as a bona fide type I IFN in hr-HPV-positive keratinocytes. RNA interference (RNAi) and cotransfection experiments indicated that the inhibition of viral transcription is mainly due to the induction of Sp100 proteins by IFN-κ. Consistent with published data showing that Sp100 acts as a restriction factor for HPV18 infection, our results suggest that hr-HPV target IFN-κ to prevent Sp100 expression and identify Sp100 as an ISG with anti-HPV activity.

IMPORTANCE

High-risk HPV can establish persistent infections which may progress to anogenital cancers. hr-HPV interfere with the expression of interferon (IFN)-stimulated genes (ISGs), which is due to reduced levels of IFN-κ, an IFN that is constitutively expressed in human keratinocytes. This study reveals that IFN-κ rapidly inhibits HPV transcription and that this is due to the induction of Sp100 proteins. Thus, Sp100 represents an ISG for hr-HPV.

Human macrophage SCN5A activates an innate immune signaling pathway for antiviral host defense

Pattern recognition receptors contain a binding domain for pathogen-associated molecular patterns coupled to a signaling domain that regulates transcription of host immune response genes. Here, a novel mechanism that links pathogen recognition to channel activation and downstream signaling is proposed. We demonstrate that an intracellular sodium channel variant, human macrophage SCN5A, initiates signaling and transcription through a calcium-dependent isoform of adenylate cyclase, ADCY8, and the transcription factor, ATF2. Pharmacological stimulation with a channel agonist or treatment with cytoplasmic poly(I:C), a mimic of viral dsRNA, activates this pathway to regulate expression of SP100-related genes and interferon β. Electrophysiological analysis reveals that the SCN5A variant mediates nonselective outward currents and a small, but detectable, inward current. Intracellular poly(I:C) markedly augments an inward voltage-sensitive sodium current and inhibits the outward nonselective current. These results suggest human macrophage SCN5A initiates signaling in an innate immune pathway relevant to antiviral host defense. It is postulated that SCN5A is a novel pathogen sensor and that this pathway represents a channel activation-dependent mechanism of transcriptional regulation.

New splice site acceptor mutation in AIRE gene in autoimmune polyendocrine syndrome type 1

Autoimmune polyglandular syndrome type 1 (APS-1, OMIM 240300) is a rare autosomal recessive disorder, characterized by the presence of at least two of three major diseases: hypoparathyroidism, Addison's disease, and chronic mucocutaneous candidiasis. We aim to identify the molecular defects and investigate the clinical and mutational characteristics in an index case and other members of a consanguineous family. We identified a novel homozygous mutation in the splice site acceptor (SSA) of intron 5 (c.653-1G>A) in two siblings with different clinical outcomes of APS-1. Coding DNA sequencing revealed that this AIRE mutation potentially compromised the recognition of the constitutive SSA of intron 5, splicing upstream onto a nearby cryptic SSA in intron 5. Surprisingly, the use of an alternative SSA entails the uncovering of a cryptic donor splice site in exon 5. This new transcript generates a truncated protein (p.A214fs67X) containing the first 213 amino acids and followed by 68 aberrant amino acids. The mutation affects the proper splicing, not only at the acceptor but also at the donor splice site, highlighting the complexity of recognizing suitable splicing sites and the importance of sequencing the intron-exon junctions for a more precise molecular diagnosis and correct genetic counseling. As both siblings were carrying the same mutation but exhibited a different APS-1 onset, and one of the brothers was not clinically diagnosed, our finding highlights the possibility to suspect mutations in the AIRE gene in cases of childhood chronic candidiasis and/or hypoparathyroidism otherwise unexplained, especially when the phenotype is associated with other autoimmune diseases.

Sp100 isoform-specific regulation of human adenovirus 5 gene expression

Promyelocytic leukemia nuclear bodies (PML-NBs) are nuclear structures that accumulate intrinsic host factors to restrict viral infections. To ensure viral replication, these must be limited by expression of viral early regulatory proteins that functionally inhibit PML-NB-associated antiviral effects. To benefit from the activating capabilities of Sp100A and simultaneously limit repression by Sp100B, -C, and -HMG, adenoviruses (Ads) employ several features to selectively and individually target these isoforms. Ads induce relocalization of Sp100B, -C, and -HMG from PML-NBs prior to association with viral replication centers. In contrast, Sp100A is kept at the PML tracks that surround the newly formed viral replication centers as designated sites of active transcription. We concluded that the host restriction factors Sp100B, -C, and -HMG are potentially inactivated by active displacement from these sites, whereas Sp100A is retained to amplify Ad gene expression. Ad-dependent loss of Sp100 SUMOylation is another crucial part of the virus repertoire to counteract intrinsic immunity by circumventing Sp100 association with HP1, therefore limiting chromatin condensation. We provide evidence that Ad selectively counteracts antiviral responses and, at the same time, benefits from PML-NB-associated components which support viral gene expression by actively recruiting them to PML track-like structures. Our findings provide insights into novel strategies for manipulating transcriptional regulation to either inactivate or amplify viral gene expression.

IMPORTANCE

We describe an adenoviral evasion strategy that involves isoform-specific and active manipulation of the PML-associated restriction factor Sp100. Recently, we reported that the adenoviral transactivator E1A targets PML-II to efficiently activate viral transcription. In contrast, the PML-associated proteins Daxx and ATRX are inhibited by early viral factors. We show that this concept is more intricate and significant than originally believed, since adenoviruses apparently take advantage of specific PML-NB-associated proteins and simultaneously inhibit antiviral measures to maintain the viral infectious program. Specifically, we observed Ad-induced relocalization of the Sp100 isoforms B, C, and HMG from PML-NBs juxtaposed with viral replication centers. In contrast, Sp100A is retained at Ad-induced PML tracks that surround the newly formed viral replication centers, acting as designated sites of active transcription. The host restriction factors Sp100B, -C, and -HMG are potentially inactivated by active displacement from these sites, whereas Sp100A is retained to amplify Ad gene expression.

Components of promyelocytic leukemia nuclear bodies (ND10) act cooperatively to repress herpesvirus infection

Upon the entry of the viral genome into the nucleus, herpes simplex virus type 1 (HSV-1) gene expression is rapidly repressed by constitutively expressed cellular proteins. This intrinsic antiviral defense is normally counteracted by ICP0, which allows virus infection to proceed efficiently. Replication of ICP0-null mutant HSV-1, however, is severely repressed by mechanisms that are conferred, at least in part, by nuclear domain 10 (ND10) components, including hDaxx, the promyelocytic leukemia (PML) protein, and Sp100. To investigate if these ND10 components repress viral gene expression in a cooperative manner, we simultaneously depleted host cells for hDaxx, PML, and Sp100 by multiple short hairpin RNA (shRNA) knockdown from a single lentivirus vector. We found that replication and gene expression of ICP0-null mutant HSV-1 were cooperatively repressed by hDaxx, PML, and Sp100 immediately upon infection, and all stages of virus replication were inhibited. Plaque-forming efficiency was enhanced at least 50-fold in the triple-depleted cells, a much larger increase than achieved by depletion of any single ND10 protein. Similar effects were also observed during infection of triple-depleted cells with human cytomegalovirus (HCMV). Moreover, using a cell culture model of quiescent infection, we found that triple depletion resulted in a much larger number of viral genomes escaping repression. However, triple depletion was unable to fully overcome the ICP0-null phenotype, implying the presence of additional repressive host factors, possibly components of the SUMO modification or DNA repair pathways. We conclude that several ND10 components cooperate in an additive manner to regulate HSV-1 and HCMV infection.

Kaposi's sarcoma-associated herpesvirus latency-associated nuclear antigen and angiogenin interact with common host proteins, including annexin A2, which is essential for survival of latently infected cells

Kaposi's sarcoma-associated herpesvirus (KSHV) infection and latency-associated nuclear antigen (LANA-1) upregulate the multifunctional protein angiogenin (ANG). Our studies demonstrate that silencing ANG or inhibiting its nuclear translocation downregulates KSHV LANA-1 expression and ANG is necessary for KSHV latency, anti-apoptosis and angiogenesis (Sadagopan et al., J. Virol. 83:3342-3364, 2009; Sadagopan et al., J Virol. 85:2666-2685, 2011). Here we show that LANA-1 interacts with ANG and colocalizes in latently infected endothelial telomerase-immortalized human umbilical vein endothelial (TIVE-LTC) cells. Mass spectrometric analyses of TIVE-LTC proteins immunoprecipitated by anti-LANA-1 and ANG antibodies identified 28 common cellular proteins such as ribosomal proteins, structural proteins, tRNA synthetases, metabolic pathway enzymes, chaperons, transcription factors, antioxidants, and ubiquitin proteosome proteins. LANA-1 and ANG interaction with one of the proteins, annexin A2, was validated. Annexin A2 has been shown to play roles in cell proliferation, apoptosis, plasmin generation, exocytosis, endocytosis, and cytoskeleton reorganization. It is also known to associate with glycolytic enzyme 3-phosphoglyceratekinase in the primer recognition protein (PRP) complex that interacts with DNA polymerase α in the lagging strand of DNA during replication. A higher level of annexin A2 is expressed in KSHV+ but not in Epstein-Barr virus (EBV)+ B-lymphoma cell lines. Annexin A2 colocalized with several LANA-1 punctate spots in KSHV+ body cavity B-cell lymphoma (BCBL-1) cells. In triple-staining analyses, we observed annexin A2-ANG-LANA-1, annexin A2-ANG, and ANG-LANA-1 colocalizations. Annexin A2 appeared as punctate nuclear dots in LANA-1-positive TIVE-LTC cells. In LANA-1-negative TIVE-LTC cells, annexin A2 was detected predominately in the cytoplasm, with some nuclear spots, and colocalization with ANG was observed mostly in the cytoplasm. Annexin A2 coimmunoprecipitated with LANA-1 and ANG in TIVE-LTC and BCBL-1 cells and with ANG in 293T cells independent of LANA-1. This suggested that annexin A2 forms a complex with LANA-1 and ANG as well as a separate complex with ANG. Silencing annexin A2 in BCBL-1 cells resulted in significant cell death, downregulation of cell cycle-associated Cdk6 and of cyclin D, E, and A proteins, and downregulation of LANA-1 and ANG expression. No effect was seen in KSHV⁻ lymphoma (BJAB and Ramos) and 293T cells. These studies suggest that LANA-1 association with annexin A2/ANG could be more important than ANG association with annexin A2, and KSHV probably uses annexin A2 to maintain the viability and cell cycle regulation of latently infected cells. Since the identified LANA-1- and ANG-interacting common cellular proteins are hitherto unknown to KSHV and ANG biology, this offers a starting point for further analysis of their roles in KSHV biology, which may lead to identification of potential therapeutic targets to control KSHV latency and associated malignancies.

Human cytomegalovirus infection causes degradation of Sp100 proteins that suppress viral gene expression

The interferon-inducible Sp100 proteins are thought to play roles in the chromatin pathway and in transcriptional regulation. Sp100A, the smallest isoform, is one of the major components of PML nuclear bodies (NBs) that exhibit intrinsic antiviral activity against several viruses. Since PML NBs are disrupted by the immediate-early 1 (IE1) protein during human cytomegalovirus (HCMV) infection, the modulation of Sp100 protein expression or activity during infection has been suggested. Here, we show that Sp100 proteins are lost largely in the late stages of HCMV infection. This event required viral gene expression and involved posttranscriptional control. The mutant virus with deletion of the sequence for IE1 (CR208) did not have Sp100 loss. In CR208 infection, PML depletion by RNA interference abrogated the accumulation of SUMO-modified Sp100A and of certain high-molecular-weight Sp100 isoforms but did not significantly affect unmodified Sp100A, suggesting that the IE1-induced disruption of PML NBs is not sufficient for the complete loss of Sp100 proteins. Sp100A loss was found to require proteasome activity. Depletion of all Sp100 proteins by RNA silencing enhanced HCMV replication and major IE (MIE) gene expression. Sp100 knockdown enhanced the acetylation level of histones associated with the MIE promoter, demonstrating that the repressive effect of Sp100 proteins may involve, at least in part, the epigenetic control of the MIE promoter. Sp100A was found to interact directly with IE1 through the N-terminal dimerization domain. These findings indicate that the IE1-dependent loss of Sp100 proteins during HCMV infection may represent an important requirement for efficient viral growth.

Human cytomegalovirus immediate-early gene expression is restricted by the nuclear domain 10 component Sp100

Nuclear domains 10 (ND10s) are discrete subnuclear structures that contain the three major protein components promyelocytic leukaemia protein (PML), hDaxx and Sp100. Previous studies identified the ND10-components PML and hDaxx as cellular restriction factors that independently counteract human cytomegalovirus (HCMV) infection via the repression of viral immediate-early (IE) gene expression. Consequently, we asked whether Sp100 is likewise involved in this repressive activity. Infection of Sp100 knockdown (kd) cells with HCMV resulted in a significantly increased plaque-forming ability. In addition, ablation of Sp100 led to a considerable increase in the number of IE1-expressing cells, indicating that Sp100 suppresses the initiation of viral gene expression. Next, double-kd cells, lacking either Sp100/hDaxx or Sp100/PML, were generated. Here, infection resulted in an additional enhancement in HCMV replication efficacy compared with the single-kd cells. Thus, our results further strengthen the concept that the three major ND10-components independently contribute to the cellular restriction of HCMV replication.

Functional reorganization of promyelocytic leukemia nuclear bodies during BK virus infection

BK virus (BKV) is the causative agent for polyomavirus-associated nephropathy, a severe disease found in renal transplant patients due to reactivation of a persistent BKV infection. BKV replication relies on the interactions of BKV with many nuclear components, and subnuclear structures such as promyelocytic leukemia nuclear bodies (PML-NBs) are known to play regulatory roles during a number of DNA virus infections. In this study, we investigated the relationship between PML-NBs and BKV during infection of primary human renal proximal tubule epithelial (RPTE) cells. While the levels of the major PML-NB protein components remained unchanged, BKV infection of RPTE cells resulted in dramatic alterations in both the number and the size of PML-NBs. Furthermore, two normally constitutive components of PML-NBs, Sp100 and hDaxx, became dispersed from PML-NBs. To define the viral factors responsible for this reorganization, we examined the cellular localization of the BKV large tumor antigen (TAg) and viral DNA. TAg colocalized with PML-NBs during early infection, while a number of BKV chromosomes were adjacent to PML-NBs during late infection. We demonstrated that TAg alone was not sufficient to reorganize PML-NBs and that active viral DNA replication is required. Knockdown of PML protein did not dramatically affect BKV growth in culture. BKV infection, however, was able to rescue the growth of an ICP0-null herpes simplex virus 1 mutant whose growth defect was partially due to its inability to disrupt PML-NBs. We hypothesize that the antiviral functions of PML-NBs are inactivated through reorganization during normal BKV infection.

BK virus (BKV) is a human pathogen that causes severe diseases, including polyomavirus-associated nephropathy in kidney transplant patients and hemorrhagic cystitis in bone marrow transplant recipients. How BKV replication is regulated and the effects of a lytic BKV infection on host cells at the molecular level are not well understood. Currently, there is no specific antiviral treatment for BKV-associated disease, and a better understanding of the complete life cycle of the virus is necessary. Here, we report the interplay between BKV and one of the regulatory structures in the host cell nucleus, promyelocytic leukemia nuclear bodies (PML-NBs). Our results show that BKV infection reorganizes PML-NBs as a strategy to inactivate the negative functions of PML-NBs.

Sp100 as a potent tumor suppressor: accelerated senescence and rapid malignant transformation of human fibroblasts through modulation of an embryonic stem cell program

Identifying the functions of proteins, which associate with specific subnuclear structures, is critical to understanding eukaryotic nuclear dynamics. Sp100 is a prototypical protein of ND10/PML nuclear bodies, which colocalizes with Daxx and the proto-oncogenic PML. Sp100 isoforms contain SAND, PHD, Bromo, and HMG domains and are highly sumoylated, all characteristics suggestive of a role in chromatin-mediated gene regulation. A role for Sp100 in oncogenesis has not been defined previously. Using selective Sp100 isoform-knockdown approaches, we show that normal human diploid fibroblasts with reduced Sp100 levels rapidly senesce. Subsequently, small rapidly dividing Sp100 minus cells emerge from the senescing fibroblasts and are found to be highly tumorigenic in nude mice. The derivation of these tumorigenic cells from the parental fibroblasts is confirmed by microsatellite analysis. The small rapidly dividing Sp100 minus cells now also lack ND10/PML bodies, and exhibit genomic instability and p53 cytoplasmic sequestration. They have also activated MYC, RAS, and TERT pathways and express mesenchymal to epithelial transdifferentiation (MET) markers. Reintroduction of expression of only the Sp100A isoform is sufficient to maintain senescence and to inhibit emergence of the highly tumorigenic cells. Global transcriptome studies, quantitative PCR, and protein studies, as well as immunolocalization studies during the course of the transformation, reveal that a transient expression of stem cell markers precedes the malignant transformation. These results identify a role for Sp100 as a tumor suppressor in addition to its role in maintaining ND10/PML bodies and in the epigenetic regulation of gene expression.

Interplay between Herpesvirus Infection and Host Defense by PML Nuclear Bodies

In recent studies we and others have identified the cellular proteins PML, hDaxx, and Sp100, which form a subnuclear structure known as nuclear domain 10 (ND10) or PML nuclear bodies (PML-NBs), as host restriction factors that counteract herpesviral infections by inhibiting viral replication at different stages. The antiviral function of ND10, however, is antagonized by viral regulatory proteins (e.g., ICP0 of herpes simplex virus; IE1 of human cytomegalovirus) which induce either a modification or disruption of ND10. This review will summarize the current knowledge on how viral replication is inhibited by ND10 proteins. Furthermore, herpesviral strategies to defeat this host defense mechanism are discussed.

Complementation of a herpes simplex virus ICP0 null mutant by varicella-zoster virus ORF61p

The herpes simplex virus (HSV) ICP0 protein acts to overcome intrinsic cellular defenses that repress viral alpha gene expression. In that vein, viruses that have mutations in ICP0's RING finger or are deleted for the gene are sensitive to interferon, as they fail to direct degradation of promyelocytic leukemia protein (PML), a component of host nuclear domain 10s. While varicella-zoster virus is also insensitive to interferon, ORF61p, its ICP0 ortholog, failed to degrade PML. A recombinant virus with each coding region of the gene for ICP0 replaced with sequences encoding ORF61p was constructed. This virus was compared to an ICP0 deletion mutant and wild-type HSV. The recombinant degraded only Sp100 and not PML and grew to higher titers than its ICP0 null parental virus, but it was sensitive to interferon, like the virus from which it was derived. This analysis permitted us to compare the activities of ICP0 and ORF61p in identical backgrounds and revealed distinct biologic roles for these proteins.

Components of nuclear domain 10 bodies regulate varicella-zoster virus replication

PML, Sp100, and Daxx are proteins that normally reside within nuclear domains 10 (ND10s). They associate with DNA virus genomes and repress the very early stages of the DNA virus replication cycle. Virus-encoded proteins counteract this innate antiviral response. ICP0, a herpes simplex virus (HSV) immediate-early protein, is necessary and sufficient to dissociate ND10s and target their two major components, PML and Sp100, for proteasomal degradation. In this report, we show that ORF61p, the varicella-zoster virus (VZV) ortholog of ICP0, does not degrade PML and alters Sp100 levels only slightly. Furthermore, we demonstrate that other virus proteins cannot substitute for this lack of function during infection. By using short interfering RNAs, we depleted PML, Sp100, and Daxx and studied their roles in plaquing efficiency, virus protein accumulation, infectious-center titer, and virus spread. The results of these studies show that components of ND10s can accelerate VZV replication but do not ultimately control cell-associated virus titers. We conclude that while both ICP0 and ORF61p activate virus gene expression, they modulate host innate repression mechanisms in two different ways. As a result, HSV and VZV commandeer their host cells by distinct mechanisms to ensure their replication and spread.

New insights into the role of the subnuclear structure ND10 for viral infection

Nuclear domains 10 (ND10), alternatively termed PML nuclear bodies (PML-NBs) or PML oncogenic domains (PODs), have been discovered approximately 15 years ago as a nuclear substructure that is targeted by a variety of viruses belonging to different viral families. This review will summarize the most important structural and functional characteristics of ND10 and its major protein constituents followed by a discussion of the current view regarding the role of this subnuclear structure for various DNA and RNA viruses with an emphasis on herpesviruses. It is concluded that accumulating evidence argues for an involvement of ND10 in host antiviral defenses either via mediating an intrinsic immune response against specific viruses or via acting as a component of the cellular interferon pathway.

Live cell dynamics of promyelocytic leukemia nuclear bodies upon entry into and exit from mitosis

Promyelocytic leukemia nuclear bodies (PML NBs) have been proposed to be involved in tumor suppression, viral defense, DNA repair, and/or transcriptional regulation. To study the dynamics of PML NBs during mitosis, we developed several U2OS cell lines stably coexpressing PML-enhanced cyan fluorescent protein with other individual marker proteins. Using three-dimensional time-lapse live cell imaging and four-dimensional particle tracking, we quantitatively demonstrated that PML NBs exhibit a high percentage of directed movement when cells progressed from prophase to prometaphase. The timing of this increased dynamic movement occurred just before or upon nuclear entry of cyclin B1, but before nuclear envelope breakdown. Our data suggest that entry into prophase leads to a loss of tethering between regions of chromatin and PML NBs, resulting in their increased dynamics. On exit from mitosis, Sp100 and Fas death domain-associated protein (Daxx) entered the daughter nuclei after a functional nuclear membrane was reformed. However, the recruitment of these proteins to PML NBs was delayed and correlated with the timing of de novo PML NB formation. Together, these results provide insight into the dynamic changes associated with PML NBs during mitosis.

Replication of ICP0-null mutant herpes simplex virus type 1 is restricted by both PML and Sp100

Herpes simplex virus type 1 (HSV-1) mutants that fail to express the viral immediate-early protein ICP0 have a pronounced defect in viral gene expression and plaque formation in limited-passage human fibroblasts. ICP0 is a RING finger E3 ubiquitin ligase that induces the degradation of several cellular proteins. PML, the organizer of cellular nuclear substructures known as PML nuclear bodies or ND10, is one of the most notable proteins that is targeted by ICP0. Depletion of PML from human fibroblasts increases ICP0-null mutant HSV-1 gene expression, but not to wild-type levels. In this study, we report that depletion of Sp100, another major ND10 protein, results in a similar increase in ICP0-null mutant gene expression and that simultaneous depletion of both proteins complements the mutant virus to a greater degree. Although chromatin assembly and modification undoubtedly play major roles in the regulation of HSV-1 infection, we found that inhibition of histone deacetylase activity with trichostatin A was unable to complement the defect of ICP0-null mutant HSV-1 in either normal or PML-depleted human fibroblasts. These data lend further weight to the hypothesis that ND10 play an important role in the regulation of HSV-1 gene expression.

Comparison of two ELISA assays for anti-Sp100 determination

Antibodies to Sp100 have been described not only in primary biliary cirrhosis (PBC), but also in other diseases. Two assays for detection of Sp100 levels by enzyme-linked immunosorbent assay (ELISA) have been compared in a cohort of patients from our area: (a) Sp100 kit produced by IMTEC, Immunodiagnostica GmbH, and (b) Quanta Lite Sp100 kit produced by INOVA Diagnostics. We analyze here the correlation between the two assays and compare their efficiency in diagnosing PBC. We also comment on the exceptions derived from reactivity with other diseases. We studied 78 sera by IIF with the typical multiple nuclear dots (MND) pattern from patients who suffered from PBC, hepatopathies different from PBC, systemic lupus erythematosus (SLE), other connective tissue diseases (CTD), skeletal diseases, lung diseases, hematological disorders, a miscellaneous group, and a healthy IIF negative control group. The tests work equally well despite their different quantification system: (a) it is based on a standard curve; and (b) it is based on a single-point antigen-specific calibration. Some discrepancies could be explained by differences in the immunodominant epitope used in the ELISA. The main finding of this study is that the presence of MND/Sp100-positive antibodies were detected not only in hepatic diseases, mainly PBC, but also in other clinical conditions, confirmed by both tests. Diagnosis of PBC must be established in the right clinical context, because other diseases recognizing the same epitope, mainly SLE, may also show high Sp100 levels. Sera from PBC patients with antimitochondrial antibodies (AMA) showed higher anti-Sp100 than the AMA-negative group.

PML and PML nuclear bodies: implications in antiviral defence

The establishment of an intracellular antiviral state is the defining activity of interferons (IFNs) as well as the property that permitted their discovery. Several pathways have been implicated in resistance to viral infection in IFN-treated cells, one of which implicates the ProMyelocytic Leukaemia (PML) protein and PML nuclear bodies (NBs, also known as ND10). PML NBs are dynamic intranuclear structures that require PML for their formation and which harbour numerous other transiently or permanently localised proteins. PML is expressed as a family of isoforms (PML I-VII) as a result of alternative splicing, most of which are found in the nucleus. IFN treatment directly induces transcription of the genes encoding both PML and Sp100, (another major component of PML NBs), resulting in higher levels of expression of these proteins and increases in both the size and number of PML NBs. These and other observations have encouraged the hypothesis that PML, PML NBs and a number of other constituents of these structures are involved in host antiviral defences. For example, exogenous expression of PML III or PML VI can impede infection by a number of RNA and DNA viruses, and certain viral proteins accumulate in PML NBs then cause their disruption by a variety of mechanisms. Although there are many other functions of PML NBs in a wide range of cellular pathways, there is accumulating evidence that they represent preferential targets for viral infections and that PML plays a role in the mechanism of the antiviral action of IFN. This article reviews the potential antiviral activities of PML NB constituent proteins, how RNA and DNA viruses overcome these defences, and the connections between these events and IFN pathways.

SP100B, a repressor of gene expression preferentially binds to DNA with unmethylated CpGs

SP100A and SP100B are mammalian nuclear proteins encoded by alternatively-spliced transcripts from the SP100 gene. The N-terminal portion of SP100B (aa 1-476) is identical to SP100A and contains an HP1 interaction domain. The C-terminal portion of SP100B (aa 477-688) contains an HMG2 interaction domain and a SAND domain. The SAND domain is a DNA-binding domain identified in several nuclear proteins involved in transcriptional regulation. We have previously reported that SP100B represses expression of genes present on transfected DNA in a SAND domain-dependent manner. The goal of the present study was to characterize the DNA binding properties of full-length SP100B expressed in mammalian cells. SP100B associated with DNA whereas SP100A did not. The SP100B SAND domain was essential for DNA binding. Deletion of the HP1- or HMG2-binding domain had no effect on DNA binding. SP100B preferentially associated with sequences containing CpG dinucleotides. Our results did not reveal any preference of SP100B for bases flanking CpG dinucleotides. The number of CpGs in a DNA sequence and spacing between CpGs influenced SP100B binding, suggesting that multimers of SP100B bind DNA in a cooperative manner. Binding of SP100B was abrogated by methylation of the cytosine residue within the context of the CpG dinucleotide. We propose that the preference of SP100B for non-methylated CpGs provides a mechanism to target SP100B to foreign DNA, including plasmid DNA or viral DNA genomes, most of which are hypomethylated.

PML contributes to a cellular mechanism of repression of herpes simplex virus type 1 infection that is inactivated by ICP0

Promyelocytic leukemia (PML) nuclear bodies (also known as ND10) are nuclear substructures that contain several proteins, including PML itself, Sp100, and hDaxx. PML has been implicated in many cellular processes, and ND10 are frequently associated with the replicating genomes of DNA viruses. During herpes simplex virus type 1 (HSV-1) infection, the viral regulatory protein ICP0 localizes to ND10 and induces the degradation of PML, thereby disrupting ND10 and dispersing their constituent proteins. ICP0-null mutant viruses are defective in PML degradation and ND10 disruption, and concomitantly they initiate productive infection very inefficiently. Although these data are consistent with a repressive role for PML and/or ND10 during HSV-1 infection, evidence in support of this hypothesis has been inconclusive. By use of short interfering RNA technology, we demonstrate that depletion of PML increases both gene expression and plaque formation by an ICP0-negative HSV-1 mutant, while having no effect on wild-type HSV-1. We conclude that PML contributes to a cellular antiviral repression mechanism that is countered by the activity of ICP0.

SP100B is a repressor of gene expression

Mammalian cell nuclei exhibit discrete sites where specific proteins characteristically localize. PML nuclear bodies (PML NBs) (nuclear domain 10s (ND10s)) are the primary localization site for the promyelocytic leukemia (PML) protein and the SP100 autoantigen. The observations that some PML and SP100 isoforms can function as transcriptional regulators, that both the size and number of PML bodies increase in response to interferon treatment, and that many mammalian viruses encode proteins that mediate disruption of PML bodies suggest that these sites suppress viral infection, perhaps by repressing viral gene expression. We hypothesized that a component of PML NBs functions as a repressor of gene expression. To test this hypothesis, we characterized the effect of PML or SP100 isoforms on expression of transfected reporter genes. PML-I, PML-VI, and SP100A did not repress reporter gene expression. In contrast, SP100B repressed reporter gene expression, especially under conditions in which the reporter gene expression was elevated by a viral transactivator or addition of trichostatin A to the culture medium. The SP100B DNA binding domain was required for repression. SP100B had no detectable effect on the amount, methylation pattern, or topological form of plasmid DNA in the nuclei of transfected cells. The demonstrated repressive activity of SP100B supports the hypothesis that SP100B is a component of an innate immune response that represses expression of ectopic DNA.

Characterization and clinical impact of antinuclear antibodies in primary biliary cirrhosis

OBJECTIVES

The clinical impact of antinuclear antibodies in primary biliary cirrhosis is uncertain. We analyzed in detail the antinuclear antibodies reactivity of primary biliary cirrhosis patients and correlated the fine specificities observed with clinical, biochemical, and immunologic parameters.

METHODS

A total of 96 consecutive primary biliary cirrhosis patients and 283 pathologic controls were studied. To dissect the fine antinuclear antibodies specificities we used different techniques, such as indirect immunofluorescence on cryostat tissue sections and cell culture (HEp-2 cells), counterimmunoelectrophoresis with thymus and spleen extracts, ELISA assays with recombinant Sp100 and purified gp210 and Lamin B receptor, and immunoblot with several recombinant nuclear and cytoplasmic antigens.

RESULTS

Antinuclear antibodies were detected in 53% of patients, with the following hierarchy of specificities: 27% anti-Sp100, 16% "multiple nuclear dots," 16% anti-gp210, 16% anti-centromere, 7% XR1, 6% anti-lamin B receptor, 5% anti-SS-A/Ro, 5% anti-ribonucleoprotein, 4% XR2, 2% anti-SS-B/La, 2% perinuclear antineutrophil cytoplasmic antibodies, and 1% anti-double-stranded deoxyribonucleic acid. Several patients showed multiple specificities. The "multiple nuclear dots" pattern was detected more often in antimitochondrial antibodies negative patients. In particular, primary biliary cirrhosis specific antinuclear antibodies (anti-Sp100, anti-gp210, and anti-lamin B receptor) were detected in nine of 13 antimitochondrial negative primary biliary cirrhosis cases. Anti-gp210 was more frequent in patients with more pronounced cholestasis and more impaired liver function.

CONCLUSIONS

Antinuclear antibodies reactivities are present in more than half of primary biliary cirrhosis patients and target diverse autoantigens located in distinct subnuclear structures. Anti-gp210 identifies a subgroup of primary biliary cirrhosis patients with more serious liver disease. Positivity for anti-Sp100, anti-gp210, and anti-lamin B receptor, either alone or in combination, may act as a serologic marker of antimitochondrial antibodies negative primary biliary cirrhosis.

Autoimmune regulator: from loss of function to autoimmunity

The autoimmune regulator (AIRE) is a gene where mutations cause the recessively inherited disorder called autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) or autoimmune polyendocrinopathy syndrome type 1 (APS1). Variable combinations of autoimmune endocrine diseases such as Addison's disease, hypoparathyroidism, and type 1 diabetes characterize APECED. The AIRE protein has several domains indicative of a transcriptional regulator. AIRE contains two PHD (plant homeodomain) type zinc fingers, four nuclear receptor binding LXXLL motifs, a putative DNA-binding domain named SAND and, in addition, a highly conserved N-terminal domain similar to the homogenously staining region domain of the Sp100 protein. At the subcellular level, AIRE is expressed in nuclear dots resembling promyelocytic leukemia nuclear bodies, which are associated with several transcriptionally active proteins. AIRE is primarily expressed in thymic medullary epithelial cells and monocyte-dendritic cells in the thymus but also in a rare subset of cells in the lymph nodes, spleen and fetal liver. The disease, caused by mutations in AIRE, its function as a protein involved in transcription, and its restricted expression in cells important in negative selection, all together suggest that AIRE is a central protein in the maintenance of immune tolerance. In this review of the recent literature we discuss the results of these studies with particular attention on the AIRE expression pattern and its function as a transcriptional regulator, as well as the effects of patient mutations on the molecular characteristics of the protein.

Autoantibodies to the transcriptional factor SOX13 in primary biliary cirrhosis compared with other diseases

The molecule SOX13 was initially identified as an autoantigen (ICA12) in Type 1 diabetes. SOX13 is a member of the SOX family of transcriptional regulatory proteins that contain a high mobility group (HMG) motif with structural similarity to HMG proteins 1 and 2. Antibodies to HMG 1 and 2 occur in autoimmune diseases of the liver and in ulcerative colitis. We measured the occurrence and levels of anti-SOX13 by radioimmunoprecipitation in primary biliary cirrhosis (PBC) and other diseases, and compared frequencies with anti-HMG measured by ELISA. Anti-SOX13 was detected in 18% of patients with PBC, 13% with autoimmune hepatitis, 18% with Type 1 diabetes, at lower frequencies in other conditions including the multisystem autoimmune diseases, systemic lupus erythematosus and rheumatoid arthritis, and in 1% of normal sera. Anti-HMG1 and anti-HMG2 occurred at frequencies of 30% and 35% respectively in PBC. Serum levels of anti-SOX13 and anti-HMG correlated significantly for PBC although not for Type 1 diabetes. Anti-SOX13 in PBC may occur merely as an immune response to products of damage to parenchymal tissue, or may be illustrative of a general proclivity of transcriptional regulatory proteins to elicit autoimmune responses.

Sp100 interacts with ETS-1 and stimulates its transcriptional activity

The cell nucleus is highly organized into distinct domains that spatially separate physiological processes. One of these domains, the Sp100-promyelocytic leukemia protein nuclear body (NB), is implicated in pathological processes, such as cancer and viral infection, yet its functions remain poorly understood. We show here that Sp100 interacts physically and functionally with ETS-1 and that NB morphology is affected by ETS-1. ETS-1 is a member of the ets family of transcription factors, which are key mediators of physiological and pathological processes. We have found that Sp100 interacts with two regions of ETS-1 (domains A+B and D+E+F). ETS-1 alters NBs while remaining localized throughout the nucleus, apparently by recruitment of the core component Sp100 away from the NBs. Sp100 strongly increases ETS-1 activation of natural and ets-focused promoters, through a mechanism involving the activation (C) domain of ETS-1 in addition to the interaction domains. Sp100 acts as a novel coactivator that potentiates the activator function of ETS-1. Our results provide an important new connection between nuclear structures and an important regulator of gene expression.

SUMO: of branched proteins and nuclear bodies

SUMO belongs to a growing number of ubiquitin-like proteins that covalently modify their target proteins. Although some evidence supports a role of SUMO modification in regulating protein stability, most studied examples support a model by which SUMO alters the interaction properties of its targets, often affecting their subcellular localization behavior. Examination of the PML nuclear bodies, whose principal components are SUMO-modified, has revealed this modification to be essential for their structural and functional integrity. This and other examples thus support the view that SUMO regulates the stability not of individual proteins, but rather that of entire multiprotein complexes.

Significant correlation between the breakpoints of rare clonal aberrations in benign solid tumors and the assignment of HMGIY retropseudogenes

Recently, we described a mechanism by which a retropseudogene, during evolution, becomes an exon of a pre-existing active gene. Similar mechanisms may account for the activation of processed genes by chromosomal rearrangements in neoplasms. Because genes of the high-mobility group protein family HMGI(Y) are known to be involved in the development of a variety of benign solid tumors, it was the aim of the present study to analyze breakpoints of clonal chromosome abnormalities in that group of benign tumors for a possible correlation with retropseudogenes of the HMGIY gene. Whereas the HMGIYL1 retrospeudogene has recently been mapped to Xp22.1, we assigned a further retropseudogene by FISH to 4q13, and database research allowed us to assign a third retropseudogene to 12q24.1. Sequence analyses of these retropseudogenes revealed high-identity indices to the HMGIY gene and no frame-shift divergences. Breakpoint information was obtained from cytogenetic aberrations in uterine leiomyomas, lipomas, pleomorphic adenomas, and pulmonary chondroid hamartomas because, in all of these tumor entities, cytogenetic subgroups involving genes of the HMGI(Y) family exist. Chromosomal bands harboring HMGIY retropseudogenes were affected with a significantly higher frequency than expected under the assumption of purely randomly occurring breakages. These results support our hypothesis that HMGIY-related retropseudogenes can be affected by chromosomal rearrangements in benign human tumors.

APECED mutations in the autoimmune regulator (AIRE) gene

Autoimmune polyendocrinopathy candidiasis-ectodermal dystrophy (APECED) is a rare recessively inherited disorder caused by mutations in the AIRE (autoimmune regulator) gene. APECED is characterized by variable combinations of endocrine autoimmune diseases such as Addison's disease, hypoparathyroidism, and type 1 diabetes. The AIRE protein contains motifs suggestive of a transcription regulator and can activate transcription of a reporter gene when fused to a heterologous DNA biding domain. In this article, mutation analyses of over 200 APECED patients published by several laboratories are summarized. To date 42 different mutations have been identified. These mutations include nonsense and missense mutations, small insertions and deletions leading into frame shifts, and splice site mutations. Although mutations are spread throughout the coding region of the gene some hotspots emerge, including the more common and recurrent mutations R257X and 967-979del13bp. Some of the identified mutations have been shown to affect subcellular localization or transactivation properties of the protein, thus providing insights into the functional properties of the predicted protein motifs.

Common properties of nuclear body protein SP100 and TIF1alpha chromatin factor: role of SUMO modification

The SP100 protein, together with PML, represents a major constituent of the PML-SP100 nuclear bodies (NBs). The function of these ubiquitous subnuclear structures, whose integrity is compromised in pathological situations such as acute promyelocytic leukemia (APL) or DNA virus infection, remains poorly understood. There is little evidence for the occurrence of actual physiological processes within NBs. The two NB proteins PML and SP100 are covalently modified by the ubiquitin-related SUMO-1 modifier, and recent work indicates that this modification is critical for the regulation of NB dynamics. In exploring the functional relationships between NBs and chromatin, we have shown previously that SP100 interacts with members of the HP1 family of nonhistone chromosomal proteins and that a variant SP100 cDNA encodes a high-mobility group (HMG1/2) protein. Here we report the isolation of a further cDNA, encoding the SP100C protein, that contains the PHD-bromodomain motif characteristic of chromatin proteins. We further show that TIF1alpha, a chromatin-associated factor with homology to both PML and SP100C, is also modified by SUMO-1. Finally, in vitro experiments indicate that SUMO modification of SP100 enhances the stability of SP100-HP1 complexes. Taken together, our results suggest an association of SP100 and its variants with the chromatin compartment and, further, indicate that SUMO modification may play a regulatory role in the functional interplay between the nuclear bodies and chromatin.

Evidence for separate ND10-binding and homo-oligomerization domains of Sp100

Nuclear domains called ND10 or PML nuclear bodies consist of an aggregation of several proteins, most notably PML and Sp100. PML is essential in the nucleation and formation of ND10 as well as in the recruitment of other ND10-associated proteins such as Daxx, pRb, BLM and Sp100. In cells induced to overexpress Sp100, ND10 binding of Sp100 was saturable and excess Sp100 formed new aggregation sites devoid of other ND10-associated proteins, suggesting that homo-oligomerization is the basis for aggregation. To determine whether Sp100 binds to ND10 through hetero- or oligomerization, Sp100 deletion variants fused with GFP were transfected into cells with and without endogenous Sp100, and the localization of the GFP-labeled fragments was determined relative to ND10. Amino acids 29-152 were sufficient for deposition of the GFP-labeled fragments at ND10 in the absence of endogenous Sp100 (heterologous binding) and for self-aggregation (formation of new Sp100 deposits). None of the shorter fragments was deposited at ND10 or self-aggregated. The 29-152 amino acid fragment and some larger fragments, but not the full-size Sp100, induced elongation of ND10, which at their ends contain only Sp100, probably due to self-aggregation. By fusing a peptide consisting of the p53-binding domain from hMDM2 to the Sp100(29-152) fragment, this self-aggregation could be blocked while retaining the limited ND10 binding capacity, indicating that the Sp100 self-aggregation domain and the ND10 binding domain are separate entities. This fusion peptide was used to demonstrate the potential of ND10 to recruit p53 as a protein not usually present at this site. Such deposited p53 was protected from turnover. The capacity of ND10 to recruit Sp100 may serve primarily to reduce its availability.

Interferon coordinately inhibits the disruption of PML-positive ND10 and immediate-early gene expression by herpes simplex virus

Interferons (IFNs) are important components of the innate immune response, limiting herpes simplex virus (HSV) infection. In recombinant HSV-infected cells, IFN inhibited expression of beta-galactosidase from the immediate-early gene, ICP4, promoter. The extent of inhibition was dependent on IFN dose, IFN type, cell type, and multiplicity of infection (moi). IFN inhibited gene transcription, leading to a complete block in ICP4 promoter-driven gene expression in 90% of cells. The same IFN treatments resulted in an increase in the size and number of nuclear domain 10 (ND10) structures that stained positive by immunofluorescence for the promyelocytic leukemia (PML) protein. In cultures infected at low moi with a recombinant HSV producing ICP4 as a fusion protein with green fluorescence protein, the appearance of green fluorescence in the nucleus coincided with loss of PML-positive ND10 in the same nucleus, even in the rare ICP4-expressing IFN-treated cells. IFN-dependent inhibition was nearly complete when the immediate-early promoter was in the viral genome but was minimal when the promoter was stably integrated into the cellular genome. These data reveal that IFN can completely block viral gene expression in infected cells and that enhancement of the ND10 structure, which is the site of initiation of HSV replication, correlates with the block in viral gene expression.

Properties of the glucocorticoid modulatory element binding proteins GMEB-1 and -2: potential new modifiers of glucocorticoid receptor transactivation and members of the family of KDWK proteins

An important component of glucocorticoid steroid induction of tyrosine aminotransferase (TAT) gene expression is the glucocorticoid modulatory element (GME), which is located at -3.6 kb of the rat TAT gene. The GME both mediates a greater sensitivity to hormone, due to a left shift in the dose-response curve of agonists, and increases the partial agonist activity of antiglucocorticoids. These properties of the GME are intimately related to the binding of a heteromeric complex of two proteins (GMEB-1 and -2). We previously cloned the rat GMEB-2 as a 67-kDa protein. We now report the cloning of the other member of the GME binding complex, the 88-kDa human GMEB-1, and various properties of both proteins. GMEB-1 and -2 each possess an intrinsic transactivation activity in mammalian one-hybrid assays, consistent with our proposed model in which they modify glucocorticoid receptor (GR)-regulated gene induction. This hypothesis is supported by interactions between GR and both GMEB-1 and -2 in mammalian two-hybrid and in pull-down assays. Furthermore, overexpression of GMEB-1 and -2, either alone or in combination, results in a reversible right shift in the dose-response curve, and decreased agonist activity of antisteroids, as expected from the squelching of other limiting factors. Additional mechanistic details that are compatible with the model of GME action are suggested by the interactions in a two-hybrid assay of both GMEBs with CREB-binding protein (CBP) and the absence of histone acetyl transferase (HAT) activity in both proteins. GMEB-1 and -2 share a sequence of 90 amino acids that is 80% identical. This region also displays homology to several other proteins containing a core sequence of KDWK. Thus, the GMEBs may be members of a new family of factors with interesting transcriptional properties.

The autoimmune regulator protein has transcriptional transactivating properties and interacts with the common coactivator CREB-binding protein

Autoimmune polyendocrinopathy candidiasis ectodermal dystrophy, caused by mutations in the autoimmune regulator (AIRE) gene, is an autosomal recessive autoimmune disease characterized by the breakdown of tolerance to organ-specific antigens. The 545 amino acid protein encoded by AIRE contains several structural motifs suggestive of a transcriptional regulator and bears similarity to cellular proteins involved in transcriptional control. We show here that AIRE fused to a heterologous DNA binding domain activates transcription from a reporter promoter, and the activation seen requires the full-length protein or more than one activation domain. At the structural level AIRE forms homodimers through the NH(2)-terminal domain, and molecular modeling for this domain suggests a four-helix bundle structure. In agreement, we show that the common transcriptional coactivator CREB-binding protein (CBP) interacts with AIRE in vitro and in yeast nuclei through the CH1 and CH3 conserved domains. We suggest that the transcriptional transactivation properties of AIRE together with its interaction with CBP might be important in its function as disease-causing mutations almost totally abolish the activation effect.

The nuclear dot protein sp100, characterization of domains necessary for dimerization, subcellular localization, and modification by small ubiquitin-like modifiers

The Sp100 and promyelocytic leukemia proteins (PML) are constituents of nuclear domains, known as nuclear dots (NDs) or PML bodies, and are both covalently modified by the small ubiquitin-related protein SUMO-1. NDs play a role in autoimmunity, virus infections, and in the etiology of acute promyelocytic leukemia. To date, little is known about the function of the Sp100 protein. Here we analyzed Sp100 domains that determine its subcellular localization, dimerization, and SUMOylation. A functional nuclear localization signal and an ND-targeting region that coincides with an Sp100 homodimerization domain were mapped. Sequences similar to the Sp100 homodimerization/ND-targeting region occur in several other proteins and constitute a novel protein motif, termed HSR domain. The lysine residue of the Sp100 protein, to which SUMO-1 is covalently linked, was mapped within and may therefore modulate the previously described HP1 protein-binding site. A consensus sequence for SUMOylation of proteins in general is suggested. SUMOylation strictly depended on a functional nuclear localization signal but was not necessary for nuclear import or ND targeting. A three-dimensional structure of Sp100, which supports the mapping data and provides additional information on Sp100 structure/function relationships, was generated by computer modeling. Taken together, our studies indicate the existence of well defined Sp100 domains with functions in ND targeting, nuclear import, nuclear SUMOylation, and protein-protein interaction.